Fourth or 4 is Soft Machine's first all-instrumental album and features a mix of Free Jazz, straight-ahead Jazz, and gong-like psychedelia. It was also the last of their albums to include drummer and founding member Robert Wyatt. The album features Hugh Hopper, Mike Ratledge, Elton Dean and Wyatt.

Soft Machine were an English experimental, psychedelic, progressive rock band from Canterbury, named after the book The Soft Machine by William S. Burroughs. They were one of the central bands in the Canterbury scene. Though they achieved little commercial success, they are widely considered by critics to have been very influential in rock music.