MS-DOS 4.0x
Originally 86-DOS, written by Tim Paterson of Seattle Computer Products, DOS was a rough clone of CP/M for 8086 based hardware. Microsoft purchased it and licensed it to IBM for use with Microsoft's IBM PC language products. In 1982, Microsoft began licensing DOS to other OEMs that ported it to their custom x86 hardware and IBM PC clones.
For IBM-specific releases, please see the IBM PC-DOS product page.
Release notes
MS-DOS 4 added support for hard drive partitions up to 2GB. It used much more of the base 640K, and was somewhat buggy.
Microsoft had long planned that MS-DOS "4" would be a multitasking-capable operating system, but IBM had insisted on creating a new version of regular DOS for use with their PS/2 computers. The below versions, as released to the consumer market, are based on DOS 3.3x and IBM's PC-DOS 4.0 enhancements. Although essentially scrapped, some narrow European markets did receive Microsoft's Multitasking MS-DOS 4.0
Microsoft's initial 4.00 release (File dates 10/6/1988) was quickly followed up by 4.01, making this an uncommon version. To add to the confusion, most OEMS badged their 4.01 disks as "4" or "4.0".
Microsoft MS-DOS 4.00 (10-6-1988) (5.25-360k)
Microsoft MS-DOS 4.00 (RM Nimbus OEM) (3.5-1.44mb)
Microsoft MS-DOS 4.0X Patch Disk PD0255
Microsoft MS-DOS 4.0x Manual