Just to add some nuance: SunOS up to version 4 was strictly BSD-based with vendor enhancement. "SunOS 5" became Solaris 2, and conversely, SunOS 4 was retroactively dubbed "Solaris 1".

Solaris 2 and up were derived from System V release 4, which had actually merged the best of System V with both Xenix and BSD, so rather than being purely AT&T Unix, SVR4 was promised as the best of all worlds, with some ability to pick and choose which variety was in play, based somewhat on provision of both types of utilities in separate directories, and appropriate libraries and APIs.

SVR4, IMHO, was the best and most stable Unix, and the right choice for vendors to adopt in those days.

Small addition: there were also the x86-based Sun 386i models, running up to SunOS 4.0.2.

(The Sun 386i didn't get SunOS 4.1 nor Solaris 2, at least not at our site, where we had a few sitting around in empty cubicles, and occasionally used for random things.)

I’m not sure why you’re being downvoted except for the minor error that SunOS was available for i386 for a short while.

The SunOS -> Solaris transition is an important piece of Sun history.