This was the group's first all-instrumental album, although their previous album Third had almost completed the band's move in this direction toward instrumental jazz, and a complete abandonment of their original self-presentation as a psychedelic and progressive rock group.

Like the previous Soft Machine album, some tracks have the band augmented by additional musicians.

These include Mark Charig and Nick Evans, who had been in the septet lineup of late 1969, and Roy Babbington, who would join the band in 1973.

In 2007, Fourth was re-released as part of the series Soft Machine Remastered – The CBS Years 1970–1973.

The booklets of these re-releases contain liner notes written by Mark Powell from Esoteric Recordings about the history of Soft Machine, their musical development and as one of the first relevant bands in the so-called progressive rock scene.