We just open-sourced DOS 4 (and found binaries of Multitasking DOS 4) https://www.hanselman.com/blog/open-sourcing-dos-4
@mirabilos @shanselman A reasonable chunk of GW-BASIC was released a few years back - I remember reading an OS/2 Museum post about trying to work out which OEM versions it corresponded to, which mentioned the OEM-specifc bits were sadly missing. But there must have been a 'generic' version of that code for the retail versions of MS-DOS to include...