Krystal and Amber shared that f(x) put in a lot of effort for their latest album '4 Walls'. 

The two along with Luna and Victoria came back as a four-member group after Sulli's leave, and it seems that they made extra sure to be on the top of their game. On the recent broadcast of 'Old School', special DJ Kangin praised, "Your latest album is really good."

Krystal responded, "Thank you. All the members worked hard to participate for this album. We showed affection for the tracks by banding together and expressing our opinion." Amber also said, "Though the members were busy because of individual promotions, we met and talked sincerely as well as practiced until early morning."

Have you checked out f(x)'s MV for "4 Walls" yet?