Original source (on modern site)
After publishing open-source versions of MS-DOS years ago for versions 1.25 and 2.0, Microsoft and IBM have now announced that MS-DOS 4.0 has been open-sourced under an MIT license.
MS-DOS 4.0 is now open-source and available on GitHub. This open-sourcing of MS-DOS 4.00 is primarily done for historical purposes. The MS-DOS 4.0 code can be tested on modern PCs with the likes of the PCem and 86box emulators.
More details for those interested via the Microsoft Open-Source Blog. The MS-DOS source code can be pulled from GitHub.