MS-DOS 4.0 was developed primarily by IBM and is notorious for its many bugs and compatibility issues. MS-DOS 4.0 adds support for partitions as large as 2GB, FASTOPEN/FASTSEEK for quicker program opening and introduces DOSSHELL, a simple GUI environment. Internal code structure changes meant that there were low level compatibility issues with some programs, particularly disk utilities, and the OS consumed much more conventional memory than prior versions of DOS. Some PC clone hardware was even known to refuse to boot MS-DOS 4.0 due to the number of bugs introduced. The included EMM386.SYS was only compatible with IBM EMS boards. Microsoft would quickly take back control over development and release MS-DOS 4.01.
At the same time the main MS-DOS 4.0 was being developed, Microsoft was developing a multitasking version of DOS also called MS-DOS 4.00. However IBM's refusal to market it would eventually result in only a few European OEMs using it.
| Microsoft Operating System Versions |
| Microsoft Operating System Versions:
MS-DOS: QDOS | 1.0 | 2.0 | 2.1 | 2.11 | 3.0 | 3.1 | 3.2 | 3.21 | 3.3 | 3.3A | 3.31 | 4.0 | 4.01 | 4.01A | 5.0 | 5.0A | 6.0 | 6.2 | 6.21 | 6.22 | 7.0 | 7.1 | 8.0 Windows 1.x: 1.0 | 1.01 | 1.02 | 1.03 | 1.04 Windows 2.x: 2.0 | 2.01 | 2.1 | 2.2 Windows 3.x: 3.0 | 3.0A | 3.1 | 3.11 | 3.2 Windows for Workgroups 3.x: 3.1 | 3.11 Windows 9x: 95 | 98 | ME Windows NT: 3.1 | 3.5 | 3.51 | 4.0 | 2000 | XP | 2003 Windows NT: Vista | Longhorn Server | Blackcomb Server Windows NT: Windows 8 | Windows 10 | Windows 11 Windows CE: 1.0 | 1.1 | 2.0 | 2.1 | 2.11 | 2.12 | 3.0 | 4.0 | 4.1 | 4.2 | 5.0 Windows Embedded: NT | XP Windows Mobile: 2003 | 5.0 Previous Windows Codenames: Janus | Snowball | Kato | Daytona | Chicago | Memphis | Millennium | Cairo | Odyssey | Neptune | Whistler | Longhorn | Blackcomb |