MS-DOS 4.0 (multitasking)

MS-DOS 4.0 was a multitasking release of MS-DOS developed by Microsoft based on MS-DOS 2.0. Lack of interest from OEMs, particularly IBM, led to it being released only in a scaled-back form. It is sometimes referred to as European MS-DOS 4.0, as it was primarily used there. It should not be confused with PC DOS 4.00 or MS-DOS 4.01 and later, which did not contain the multi-tasking features.

Apricot Computers pre-announced "MS-DOS 4.0" in early 1986, and Microsoft demonstrated it in September of that year at a Paris trade show. However, only a few European OEMs, such as SMT Goupil () and International Computers Limited (ICL), actually licensed releases of the software. In particular, IBM declined the product, concentrating instead on improvements to MS-DOS 3.x and their new joint development with Microsoft to produce OS/2.

As a result, the project was scaled back, and only those features promised to particular OEMs were delivered. In September 1987, a version of multi-tasking MS-DOS 4.1 was reported to be developed for the ICL DRS Professional Workstation (PWS). No further releases were made once the contracts had been fulfilled.

In July 1988, IBM announced "IBM DOS 4.0", an unrelated product continuing from DOS 3.3 and 3.4, leading to initial conjecture that Microsoft might release it under a different version number. However, Microsoft eventually released it as "MS-DOS 4.0", with a MS-DOS 4.01 following quickly to fix issues many had reported.

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