Fourth is a 1971 studio album by the Canterbury band Soft Machine. The album is also titled Four or 4 in the USA; the numeral "4" is the title as shown on the cover in all countries, but a written-out title appears on the spine and label. 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 pop group, or progressive rock group. It was also the last of their albums to include drummer and founding member Robert Wyatt who afterwards left to record a solo album, The End of an Ear (in which he described himself on the cover as an "out of work pop singer"), and then founded a new group, Matching Mole, whose name was a pun on "Soft Machine" as pronounced in French: "Machine Molle".
Like the previous Soft Machine album, this one uses session musicians who were not regarded as full group members, but toured with the band for live performances.
In 1999, Soft Machine albums Fourth and Fifth were re-released together on one CD.
In 2007, The 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.