4 Walls is the fourth and final studio album by South Korea-based girl group f(x), released by SM Entertainment and Genie Music on October 27, 2015. The album also attained commercial success, becoming their fifth and final chart-topper on the Gaon Album Chart, and has since sold over 81,000 physical copies in South Korea as of March 2016. It also earned success for f(x) in the United States, becoming their second release to achieve No. 1 on the Billboard World Albums Chart and charted at number thirty-nine on the Japanese Oricon Albums Chart. To promote the album, its titular track was released on October 27, 2015 to both critical and commercial success, peaking at number two on both the Gaon Digital Chart and the Billboard World Digital Songs.