4 Walls is the fourth studio album by South Korea-based girl group f(x), released by SM Entertainment and Genie Music on October 27, 2015.

A predominantly electropop and synth-pop record that incorporates various genres, such as house, EDM and UK garage, 4 Walls was composed by several production teams and songwriters, namely LDN Noise, The Stereotypes, Kenzie, Ryan S. Jhun, Makeba Riddick, Carly Rae Jepsen, among others, with Lee Soo-man served as the executive producer of the release.

[1] Upon its release, 4 Walls was praised by music critics for increasingly showcasing all members' vocal abilities with "trendy" production.

The group further embarked on their first concert tour, titled Dimension 4 – Docking Station, which visited South Korea and Japan in January and February 2016, to promote the majority of the album and their past releases.

[3] During the promotion of Red Light, member Sulli did not participate in live performances due to her "suffering from continual negative comments and false rumors," according to the group's parent company S.M.

[5][6] 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.

[16] The tour kicked off in Seoul, South Korea at Olympic Park from January 29 to 31, 2016, and further included six shows in four cities of Japan: Tokyo, Fukuoka, Osaka and Nagoya.

[19][22] "Deja Vu" mixes industrial beats with glitch pop sounds, while "X" draws influences from the 1980s funk, 1990s disco and R&B.

"[24] Sheldon Pearce on behalf of Pitchfork labeled the record a "superficial rebirth" following f(x)'s lineup change and commented that the other members had the chance to express their talents more, particularly with Amber's alto "harmonies" on "Rude Love".

He nonetheless labeled them "minor misgivings" and complimented 4 Walls on featuring "sophisticated and innovative" musical styles that could "signal a surprise second life for the world's greatest living pop group.

"[19] Echoing Dorof's viewpoint, Chester Chin of The Star was not impressed by the inclusion of the "derivative" tracks "Glitter" and "Cash Me Out", yet expressed his admiration for the "solid and edgy" album's "more cohesive" sound as well as "more polished and sharper" beats.

[25] It was the fourth best-selling physical album of October 2015 in South Korea, with sales figures standing at 65,933 copies.