[spijdar]

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.

[icedchai]

Very true. SunOS 4.x is still my favorite 90's Unix. I had a Sun 3 box for a while, then got a low end Sparc Station at home! Eventually in the late 90's I gave in and installed Solaris. 2.4 and earlier was kinda rough, but it was pretty decent by 2.5.