Multitasking MS-DOS 4

Multitasking MS-DOS 4 (sometimes also referred to as European MS-DOS 4) refers to a dead-end branch of MS-DOS that included support for multitasking, which was only offered by certain European OEMs for the most part. Unlike its predecessors and even contemporary versions of Microsoft Windows, it implements proper preemptive multitasking with a time-sliced scheduler.

The project was intended to be the successor to MS-DOS 3, although it does not share much amount of code with it. Most executables are New Executable (NE) files (excluding the DOS BIOS), with a large amount of the operating system being rewritten in C, including the entire Command Interpreter and a large percentage of the DOS BIOS. The default  entry in   points to the Session Manager, which is a small shell application that provides a UI for application switching. The real shell is provided as an argument to the Session Manager.

Most components of this version of DOS were compiled on 26 November 1985, shortly after the completion of Windows 1.0. Internally, it identifies as "internal revision 6.7" a la OS/2, although the string has been patched to not display, which indicates that this may be a pre-release that was shoved out to OEMs. Goupil released an OEM version of Multitasking MS-DOS 4.0 in May 1986, and is mostly the same as the non-OEM version. An updated version, MS-DOS 4.10, was developed for the ICL DRS Professional Workstation and is known to exist in private collections.

Documents indicate many features, such as threads, were also planned but removed during development. A successor, DOS 5.0, was also under planning, which would have included threading as well as support for protected mode and installable file systems. The DOS 5.0 project eventually morphed into OS/2, to the point that Microsoft still used the term "DOS 5.0" internally to refer to the new operating system during the development of OS/2 1.0.