"4 Walls" is a song recorded by South Korean-based girl group f(x) for their fourth studio album of the same title. The track was written by Jam Factory member Lee Seu-ran, whilst production was handled by LDN Noise, Tay Jasper, and Adrian McKinnon. Lee Soo-man, the former president at S.M. Entertainment, served as the executive producer to the track. It was released on October 27, 2015 as the lead single in conjunction with the release of the album. Musically, "4 Walls" is described as a hybrid of tropical house, UK garage, and deep house.

The track garnered generally favorable reviews from music critics, most of whom highlighted as one of the album's best tracks and complimented the song's music styles. Commercially, the song peaked at number two on both South Korea's Gaon Digital Chart and United States' Billboard World Digital Songs. An accompanying music video features the members witnessing themselves in different dimensions. The group promoted the song by performing it on several Korean television shows, including Music Bank and M Countdown.

South Korean girl group f(x) had achieved success in South Korea, and through Asia with singles including "Electric Shock" and "Hot Summer" as a five-piece group, consisting of Victoria Song, Amber Liu, Luna, Krystal Jung and Sulli. During the promotion of the groups third studio album Red Light (2014), 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. Entertainment's official statement on July 25, 2014. f(x)'s promotional activities thereafter involved the four other members, while Sulli took a career hiatus. On August 7, 2015, it was announced that Sulli had officially withdrawn from the group to focus on her acting career, and f(x) would continue to promote as a four-member group.

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