[Warning: I'm sorry chacha, but this ended up harsher than I thought. I don't know why I'm being so harsh to people lately LOL. But please have some mental preparation before going into this review, because even when I read it myself, I'm shocked by my own writing :X And please always remember that this review is meant for you to improve.]

FIRST IMPRESSIONS (8/15)
Title: 2/5
`Attractiveness: 1/2
We don't know the context behind the numbers, and so personally, the title didn't really catch my eye initially. 4/7 is an interesting way to present how there are lost days, and it remains significant when the readers have read the story.
`Relevancy: 1/2
Relevant, but it's just way too vague. I thought it meant a date when I first saw it lol.
`Originality: 0/1
Numbers have been cast around very often in AFF titles, so now numbers just don't seem to mean anything anymore. Not very original, sadly.

Description & Foreword: 2/5
`Attractiveness: 1/3
'Baekhyun spared a week for you, but that's lasted four.' - I was really confused by this statement, but then I realised what you meant. You should rephrase it to this: 'Baekhyun spared seven days for you, but you and him only lasted for four.'
It's a bit lengthy. I think there are some parts which you can cut down. I suggest you cut everything from 'On...' to '...but fate said no' because the main focus of the story here seems to be on the later angsty part. I like the last two sentences, but I think for the last sentence, you meant 'Tears, memories and regrets would all be abandoned to the cold winter wind.' because if not, the last sentence contradicts with the first one. Other than that, the description sets the angst vibe really well.
`Relevancy: 1/2
Relevant, although it is unclear what spring refers to at this point of time. Also, 'winter' is not established very well.

Format (Graphics & Layout): 4/5
`Graphics
Aw, you're great with graphics, actually! It's really beautiful, and it certainly fits the angst fluff vibe very well. But with how Serri is alone in the poster, it feels like Serri is going to end up alone, and that'll just spoil the whole point of the story. I wonder what 'to eternity' means over here. What's with the clock though?
`Layout & Formatting
The layout is simple and neat, accompanying the story very well.

WRITING STYLE (8/25)
Grammar & Vocabulary: 4/10
`Grammar: 2/7
``Terminology
[T] - Tenses. [ss] - Sentence structure. [c] - Conjunction. [e] - Expression error/inappropriate expression.
Know your problems very well so that you can learn how to correct them.

``General overview
Your sentence structures are very awkward. Because you like to write long sentences, this problem is aggravated since you would string a series of ideas together or include unnecessary details. Just go straight to the point. If you see a long sentence, cut it down so that you don't confuse yourself and your readers.
Like the previous requester (and actually with many authors out there), you use present participles '--ing' a lot, but when they're used inappropriately, the sentences just don't flow very well. Also, you also join a series of sentences together without proper punctution (e.g. you should use a full stop to separate complete sentences, yet you use commas occasionally). You tend to miss out closing punctuation at the end of the dialogue (like full stops, commas etc). Sometimes you would switch between tenses from past to present, and it becomes confusing. There are also some other errors in your story. English is not your main language, so I understand you (/hugsss), but I do advise you to get a good editor to smoothen out your sentences (and you can learn from them in the meantime too). Although grammar doesn't make or break a very good story, the consistent mistakes can be very distracting for more nitpicky readers. First impressions count a lot :x

``Examples
C1: "You were standing on your feet, having a white light stick with a zero-four sticker under the logo of the group on your right hand waving it along with others. [ss]" - 'on your feet' is redundant, and it can be misleading because it sounds like you were actually standing on your own feet (just imagine that- isn't it funny? xD). The 'having' part is also redundant; you can simply just say you were waving the white lightstick. The sentence is also confusing, and since it's hard to explain how it's confusing, I just adjusted it. Compare between your original sentence and these altered sentences, and you'll realise that the altered sentence simplifies and shortens things. The only magic way to resolving your convoluted sentence structures is to read more, get a feel of how sentences should actually flow and try to apply that to your own writing. Also, go straight to the point. If you're waving a lightstick, then just say you're waving it. Try not to include unnecessary details to further complicate your sentences. "You were standing, waving a white lightstick in your right hand along with the others. There was a logo of the group on the lightstick, and a zero-four sticker was just under it."
C1: "...the gentle voice which always saying [T] the comforting yet [c] encouraging words whenever you are facing [T] some problems that make [T] you wanted to kill yourself [e]" - 'which' is unnecessary here. A complete sentence involves a subject, action and body, but if you add 'which', it makes the sentence incomplete because that makes the rest of the sentence a 'description' to the voice, and not the actual action and body themselves. Try not to use '--ing' words casually; they're used to express continuity but they shouldn't be used all the time. There is no contrast between comforting and encouraging, so the contraction 'yet' is used inappropriately here. 'kill yourself' is not a grammatical error, but it's just way too direct and harsh. I suggest avoiding the use of such sensitive words in narration. "...the gentle voice which always said comforting and encouraging words whenever you faced problems that rendered you lost and dejected." 
C1: "His lips was quivered [T]..." - You can either change 'was' to 'were' and change 'quivered' to 'quivering' to express that his lips continued to quiver or remove 'was' entirely and just keep 'quivered' as it is. The second option is probably easier. But as the sentence stands now, it's not grammatically correct.
C2: "You were half asleep, eyes were blurry [e], all over your body was aching. [ss]" - If you say 'your eyes were blurry', it literally sounds like her eyes looked blurry (which makes no sense, you see). I think you mean that her vision was blurry/hazy. The sentence structure for the later part is also a little weird. "You were half asleep. Your vision was hazy, and your body was aching all over."
C3: "Along with your pouted lips [e], Baekhyun dragged you to enter the house." - Sounds like Baekhyun dragged you and your pouted lips (which makes no sense again lol). See the magic of misleading sentences? xD Again, there's redundancy here. It's already hinted that Baekhyun and Serri had entered the house (since he dragged her into it); you don't have to repeat 'enter' again. "Although you were pouting, Baekhyun dragged you into the house."

`Vocabulary: 2/3
It's quite okay actually. There's attempts to use diverse sentence structures and words, and they're quite successful, though occasionally words are used inappropriately. But most sentence structures are awkward, which end up affecting your grammar instead. Try not to use vague words like 'cute' and 'happy', and instead try to describe it out.

Writing Style: 4/15
`Description: 2/5
I can really tell that there's effort put into describing, but it's simply not enough. You tend to describe just the basic actions but neglect the subtle details like facial expressions, 5 senses (aside from sight) and just the attention to detail. For instance, the contrast of the four days and the non-existent three days should be the main focus and strongest point of the story, but the four days are described in a very shallow manner. We don't know what exactly happened during the dates, the actual interactions between Baekhyun and Serri and anything else that can make us feel for the couple.
It would also be better if there's more usage of scenery, considering that your description and poster seem to reflect that as a major part of your story. What value do the snowflakes have on Serri? What are her views towards them? Things like these can also shape Serri's character and make her someone who's individualistic with her own beliefs, values and opinions.
C2: "You can see that Baekhyun was a little bit annoyed by the sudden call." - Don't say 'annoyed'; just show it. Did Baekhyun start hissing at the caller? Was he frowning, or did he start 'tsking' at the person? After the call, what was his expression like? Did his shoulders droop? Did he rub his head out of frustration? All these subtle details matter, and sadly, you haven't managed to showcase all of these throughout the whole story.
C2: "You can't hide your pain because of Baekhyun" - How? If Serri was in pain, then how did she show it? Did she clutch her chest like she was being suffocated? Or if she rarely showed her emotions, then what was she feeling in the inside? How were her thoughts like? How did Baekhyun's face look like? You can use exaggeration or poetic devices, (e.g. She indulged in gazing at her boyfriend's face, for his smile was like a calm breeze that soothed her dismayed heart. Yet, when she looked up at his face then, his eyes burnt with resentment towards her, and she flinched at the sight of the tornado that was reflected in his eyes) All these make a lot of difference in conveying the emotions of your characters.
There is occasional use of metaphorical writing, like "That voice, always lead you to the light, dragging you out of the sorrows and frustrations". A little cliche, but it's a step towards the right direction. You can just say 'darkness' instead of 'sorrows and frustration' for a better contrast, though.

`Emotions: 1/5
The lack of description has already placed you at a disadvantage for this section, but some authors have managed to convey sufficient emotion without much description. You're quarter-way there. I say that because truly enough, the dialogue is genuine and emotion-packed. I have to say, there are really too many curse words in the last two chapters though- I don't have problems with curse words but when you use them too much, it comes off as kinda contrived, so it's something to take note of. Dialogue can't save everything though. The narration is simply not adequate, because when the moments are right, thoughts are not written. Emotions are not shown through expressions and scenery. And even if emotions are shown, the emotions are barely grazing the surface. It's easy to say that someone's sad, happy and all, but these aren't the only emotions in the world. Humans are complex; they can experience many complicated feelings at one go, and if you're able to express this complexity out, then you've really succeeded in the emotions department. But you haven't quite achieved that yet. We know Serri was sad, but what else? Was Serri regretful? Disappointed? Angry? Jealous? Baekhyun's desperation was actually shown pretty well, although I still have doubts on his character (which I'll express later in the characters section).
Even during the sugary sweet moments, both of the characters felt extreme joy and their emotions never went beyond the surface, making them very underdeveloped characters. And since all of these elements are linked together (description, emotions and characters), all these aspects are affected by one another. And for an angst-fluff story, this is a very heavy blow. Not being able to deliver your characters' emotions well enough is a very heavy blow. The only way to improve is to merely imagine yourself in the shoes of your characters, then reflect on the possible feelings you may experience. Not just emotions like sadness, but also other emotions. Afterwards, convey that to the readers. Read your story again, and see if you can feel the feelings you wanted to portray as you read the story (try to read it without bias, and just as an objective piece). I'm sure you can do it, since you said that you want the readers to feel how you feel! And since you already have the intentions, then being able to accomplish that would become easy with sensitivity and practice.
P.S. If you ask me whether I've felt the emotions, well, yes I actually have. But as mentioned before, I feel like they're portrayed in a very shallow manner, and there's so many things you can do to make them more developed and genuine. Emotion is subjective, so it's very difficult to tackle, but you'll just have to find your own way in evoking emotions out from others somehow along your writing journey.

`Memorability: 1/5
The quotes are a nice touch. Your dialogue is somewhat playful sometimes, but it feels a little bland. It doesn't show the individual characters very well. The narration is also pretty generic and not developed enough.

DIGGING DEEPER (16/55)

Characters: 5/25
`Believability: 2/10
``Serri
Okay, so I know you said you had reasons, but I simply can't understand Serri. So, she cheated on Baekhyun... or did she? She said she did, but Hansol was in her living room and not in her bedroom so I don't think she did? o_o I don't know why she said she did that though. What was her reason? Why didn't she try to explain herself, at the very least? Why did she even let Hansol accompany her to EXO's waiting room etc? Considering Baekhyun didn't even know about her friendship with Hansol beforehand, it just seems so strange that the dude suddenly became like her best friend after the cheating incident. Because of this, she gives the readers very conflicting ideas (not to mention I don't even know if she cheated or not-). The reason why this happens is because there's no foreshadowing or explanation to it. We're just thrown into this problem all of a sudden, and we don't even know Serri and Baekhyun very well before this problem occurs (thanks to the lack of development of the 4 days). Also, it's very frustrating to read Serri as a main character mainly because she doesn't do anything. We know she has something to her; it's there, but it's not executed well enough. We're not given entry to her own world. She's like this mysterious character with a secret, but even we the readers don't know what's up with her. If you don't let us into her world, then we'll always be outsiders detached from the universe of this story. And so, this is a really major problem, since she's the main character after all.
Another thing is this. Serri's personality. Her personality is very sweet and sometimes playful (though she seems to have a hidden side) but there's no flavour to it, partially because she doesn't do anything. She only cried so far. We know she was sad, but we don't even know why she did not do anything about it. The only thing I quite appreciate about her is her unconditional love for Baekhyun, but then it brings me back to the question: Why exactly did she cheat on him??? Lol really, I'm sorry but I was literally raging when it happened, because I just can't understand it. Anyway, Serri as a character needs development on her motivations. There's must be a solid reason as to why she did something (cheating) and why she's not doing something (giving up Baekhyun quietly). And I get that she's supposed to be a self-insert, but I personally don't like self-inserts very much ; u ; I prefer if she's an independent character with her own beliefs and principles, but right now, Serri has none of that.

``Byun Baekhyun
Many people will probably disagree with Baekhyun's actions in this story, but I don't see what's the problem. He's actually quite believable considering what he had experienced and his status. I personally don't agree with his actions. However, I understand why anger consumed him and he refused to listen to his friends (he was really being a about it though). They clearly don't know what happened, and all they're doing is defending Serri without listening to his take. Obviously he would be pissed. The rebound is actually somewhat expected, though I'm surprised about the last part when Baekhyun's relationship with Taeyeon is revealed, and he was saying he didn't want to break up with Taeyeon while simply worried about his bracelet without much concern towards the other party. Comparing his attitude then to his initial attitude towards Serri (where he tried to keep their relationship secret as much as possible for Serri) just shows how he really didn't care about Taeyeon at all. And that's really shocking, in my opinion. He's said to be a very caring guy, and I can almost believe it if I read the earlier chapters, but his caringness literally just flew out of the window at the last two chapters. I get that he was traumatised by Serri's 'betrayal', but c'mon Baekhyun. That doesn't mean you should be a to others, and definitely not to your new girlfriend who has zero idea of what happened to you and your ex.

``Others
Hansol was so patient and caring towards Serri, but I wonder why he was okay with being labelled as a third party. The EXO members seem like the supportive but teasing friends, which is okay until the cheating incident happened. Clearly, they all started to take Serri's side, which is really unbelievable to me. Just because she's the main character doesn't automatically make her out to be the angel of the story, and I'm sad to say this, but this is the feeling I get right now ; n ; They knew Baekhyun longer than they knew Serri, and if they knew Serri cheated on him, how can they just... do this? I mean, Baekhyun is clearly being horrible, but I would expect more consideration on his band members' part. Why did Chanyeol know about Hansol and Serri but Baekhyun didn't? o_o It's as if Chanyeol was closer to Serri than Baekhyun...  

``BaekRi's relationship
Their interactions initially were sugary sweet and reminds me very much of the 'honeymoon period' that couples have until it all fell apart when Serri began cheating on Baekhyun. It actually seems like a pretty real relationship, especially with how Baekhyun went on rebounds. But it would make more sense if Serri's reasons were explained. The subtle care that Serri showed to Baekhyun was almost touching, but then I'm just reminded of whatever that I said just now and... /dramatic sighs. I'm so sorry chacha. ;; It's just that your weaknesses can easily overshadow your strengths, so it's very important for you to try and minimise the mistakes you make so that at the very least, the readers can still appreciate your strengths.

`Portrayal: 2/10
``Serri
I actually said most of what I wanted to say in the Believability/Characters section xD But basically, she's just... bland. I can appreciate some of her sweet and playful moments but that's it. She needs to have stronger emotions, believable reasons and inspiring convictions. 

``Byun Baekhyun
He started out as a cheesy guy, which was rather stereotypical. Then, he made a 180 degree change, and I have mixed feelings about this. Even though I understand him, his actions are too overboard. He reminds me of those really wangsty guys who start going on emo mode when their girlfriends broke up with them. They push away everybody who tries to show them concern, and it's just so hard for me to feel for him even if I understand him. Not to mention, he kept calling Serri names, labelling her as this and that, and I was... shocked. He loved her so much, but then he just acted like she was a piece of trash right after that. Like I love Baekhyun, but I'm really curious as to how he's going to redeem himself in the next few chapters, because I'm not getting a good feeling right now ;;

``Others
They're kinda funny, but pretty forgettable for the most part. I'm still wondering why they sided with Serri ;;

``BaekRi's relationship
It's okay as a whole and a pretty relatable relationship. You just have to work on the portrayal of the individuals, because right now, I don't feel for either Serri or Baekhyun :x

`Development: 1/5
``Serri
There is no development. She doesn't reflect on her actions, and she doesn't take action. I guess we just get to see how unconditional her love for Baekhyun is (though she was insulted by him and stuff), but well, it really makes no sense.

``Byun Baekhyun
Rather than developing, I guess you can say he regressed for the worse lol. The regression is done nicely, but at the same time, I wish we can seen more development in him as well.

``Others
I feel like you can actually show their emotions of conflict as bystanders of the two's relationship, but for now, they don't really have much impact.

Plot: 7/25
`Theme: 2/10
I think you have a message that you want to convey through this story, but because of the confusion I have with the plot, I can't really tell what's the main message. Is it that cheating harms relationships? Or is it that we must listen out to the other party before quickly passing judgments? Or is it about the lost days and how we should cherish every memory we have? It's quite confusing, and while I can speculate what the message is, I didn't feel like I was actually impacted by BaekRi's story. The angst, fluff and romance is well conveyed, but the angst greatly overshadows the fluff, so that's why you see me ranting about the angst in the majority of this review lol. 

`Portrayal: 3/10
If you ask me whether I can guess what's coming up for the next chapter, I really can't, and maybe that's a good thing. But the bad thing is I don't understand the current content of the story, and that really hinders me from appreciating what I might've been able to appreciate if I did understand it (Serri and the theme for instance). Mainly the cheating part is the problem. I've addressed my questions before so I won't repeat them again lol. I think what you were trying to do is create a suspenseful atmosphere, but you're not tactful enough in your approach and so it didn't come off as very convincing or consistent.
A few suggestions to make this story an even better one:
1. I have a feeling you're going to pull a plot twist. If you do it, make sure it's a convincing enough reason. My instinct tells me that Serri probably has a mental illness or something of that sort, and she wants to keep Baekhyun away from her (but I have doubts again after reading the fic one more time). Whatever the twist is, make sure it's solid and executed properly.
2. This seems to be a mini chaptered fic, but do still develop the characters' fluffier moments sufficiently. Then, when we eventually descend down to the angst, we'll try to stay for the characters even if we don't quite understand them sometimes.
3. Foreshadowing is very important. If Serri is going to cheat, leave hints before the cheating incident is going to happen so that we won't be like 'what?' when it does come. I can see attempts of foreshadowing with Serri's random question but again, it's not done tactfully enough. It's just kinda vague and somewhat gives us the idea that Serri has something bad going on, yet no thoughts and description accompanied her when she asked that question, so all in all, I'm just left with question marks instead of having a direction of where to go to lol.
I guess I can say the story is quite original in its approach, so that's good. The couple's portrayal is done well, but the combined efforts of many things (especially especially the theme) succeed in bringing the portrayal score down.

`Realism: 2/5
I think the unclear intentions of Serri brought this score down a lot. But other than that, the plot should generally be okay.

Flow: 4/5
The four days can be expanded on a little more, and better foreshadowing would make the cheating incident less abrupt than it seems to be now. Other than that, it's relatively fine.

FINALLY (1/5)
Overall Enjoyment: 1/5
I can tell you put in a lot of effort really, and I do want to love your story! But unfortunately, I just can't bring myself to, because once something I deem terribly jarring distracts me, it's very difficult for me to appreciate the rest of the story. I think you have to fix the root of the problem, Serri's unclear intentions. One thing can seriously make or break a story like this. But I did somewhat enjoy the first two chapters, even though I'm not a fan of sugary sweet relationships. 

OVERALL ANALYSIS
Strongest aspect/s: Flow, Vocabulary/Grammar & Vocabulary, Format (Graphics & Layout)
Weakest aspect/s: Characters, Overall Enjoyment
Technicality Level: 1/5 - Really, I blame Serri's unclear intentions.
Feels Level: 1/5 - All the feels I had were seriously destroyed in just three chapters, really.
Longevity Level: 2/5 - It's good to go back to for me to figure out what actually happened, but there is insufficient foreshadowing for attention to be sustained.
Memorability Level: 2.5/5 - I'll remember this, but probably not in a good way >_< I'm sorry!!

TOTAL SCORE: 33/100
Comments: This is really a classic example of how one chapter can make and break a story omg. Believe me, it happens. I read a story which I thought was fantastic for the first 40 chapters, until I came to the last 10. My love for that story was absolutely crushed then. Anyways, I can see that you put in a lot of effort for this story, but there are many areas you have to work on. First and foremost, SERRI'S UNCLEAR INTENTIONS! Seriously. Then after that, you can progressively work on the rest. Please don't feel disspirited just because of this review, since there are probably people out there who won't reject Serri's unclear intentions as much as I do. And if you feel unhappy, you can use your unhappiness as a source of strength to improve and prove me wrong haha. Anyways, I hope this review has helped, and if you have any doubts, feel free to clarify them with me. I'm more than happy to discuss the details with you again.