4 Walls is the fourth full-length album by f(x). It was released on October 27, 2015 with "4 Walls" serving as the album's title track. It marked the group's first release as a four-member group after former member Sulli left the group in August 2015.
The record's musical styles are diverse, incorporating the group's signature electropop and synthpop sound with elements of various genres including house, EDM and UK garage. The album received favorable reviews from music critics, who noted that the other members had the opportunity to showcase their vocal abilities more following the departure of Sulli.
An art exhibition to showcase the group's comeback was set up in Itaewon, Seoul and ran from October 21 to 27.
The album was a commercial success in South Korea, selling 75,625 copies in 2015 among the two months of its release and becoming the 33 biggest-selling album of the year in the country. The title track was also the group's most successful single since "Rum Pum Pum Pum", selling over 619,022 units as of June 2016.