SM娛樂
f(x) was announced to be working on its then-forthcoming studio album on September 11, 2015, when the group traveled to Jeju Island to film a music video. The album, titled "4 Walls", was released digitally worldwide on October 27, 2015 by S.M. Entertainment. In South Korea, the album was distributed digitally and physically by S.M. Entertainment and KT Music. The music video for the title track "4 Walls" was released simultaneously. The song impacted Korean Broadcasting System's "K-Pop Connection" radio on October 29.