MS-DOS 4.00 is quite an interesting version of this command-line-driven OS, as it was written in partnership with IBM and formed a branch of DOS called Multitasking DOS (or MT-DOS), that only had a limited release.
The source code Microsoft released this week is not for that multitasking version of DOS 4.00, and Microsoft’s Open Source Programs Office was “unable to find the full source code” for MT-DOS when it went to look.
This isn’t the weird multitasking variant, is it?
According to the Tom’s hardware article from another post, it looks like it is.
https://arstechnica.com/gadgets/2024/04/microsoft-and-ibm-release-source-code-for-one-of-the-weirdest-versions-of-ms-dos/