DOS 4.0 was a July 1988 update for users of older versions of MS-DOS. It included a graphical/mouse interface, and for the first time, support for FAT16 hard drives. MS-DOS 4.0 was shipped in 5.25" floppy disks, like all older versions of DOS.

Later in 1988, an improved version called "MS-DOS 4.01" was released for IBM PCs, which included fixes reported by many users of DOS 4.0.

Compaq PCs added support for MS-DOS 4.01 in April 1989.

A version 4.02 of MS-DOS was released in the Soviet Union early in 1990 with a copyright message.