MS-DOS 4.0: The Next-Gen DOS That Never Shipped
Most people don’t know about Microsoft's other multitasking operating system, MS-DOS 4.0 which was actually a version of MS-DOS 2.0 that was written in parallel with MS-DOS 3.x (DOS 3.x shipped while DOS 4 was under development, which is why it skipped a version). The MS-DOS 4 version described here was never shipped and instead a beefed-up version of 3.x was labeled 4.0 and shipped instead.