A lot of 90's stuff ran great on SunOS 4.x!

Yes, but SunOS 4 was both extremely popular (enough that a lot of software had explicit support for running on it) and implemented a decent amount of System V and POSIX compatibility!

Probably most notably, it implemented SysV shared memory (sys/shm.h) plus messages/semaphores, STREAM support, SysV termio, SysV libcurses, and probably others I'm not aware of.

I'm not sure how much any of these helped run software, but it bears pointing out anyway.