One of the people who helped archive the MS-DOS 4.0 source code, @starfrost, started writing an extensive and interesting story about the history behind 4.0 and the archived Multitasking DOS (MT-DOS) builds: https://starfrost.net/blog/001-mdos4-part-1/index.html
(the page didn't render well on mobile for me but it can be fixed by switching to "Reader view" in Firefox)
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