These disk images contain the first unearthed copy of Microsoft's "multitasking" version of MS-DOS. They were were discovered by the PCjs Project in March 2013. This version of MS-DOS was never officially released in North America. More information is available at Wikipedia and OS/2 Museum.
Note: The designation "4.0M" is not an official version number; it's an internal version number used only to differentiate these disk images from those of the official MS-DOS 4.00 release in July 1988.
The IBM Model 5160 machine configuration below boots MS-DOS 4.0M. To access the MS-DOS "Session Manager" after it's booted, tap the Alt key.
IBM Model 5160
Waiting for PCjs to load