The source code of “DOS 4” was released:

https://github.com/microsoft/MS-DOS/tree/main/v4.0

Not without issues:

https://www.os2museum.com/wp/how-not-to-release-historic-source-code/

(Hence “DOS 4” in quotes, is it 4.00 or 4.01? Probably the latter.)

More DOS 4 history:

https://www.os2museum.com/wp/dos/dos-4-0/