That was sort of complicated and I'm not sure whether the BSD4 license development was related. It's possible (I don't know) that BSD4 was developed for some other reason like the VLSI tools Berkeley was releasing at the time.
Regarding BSD itself, there was a lawsuit between AT&T (or some successor) and UC, that was settled by UC having to delete some files from the BSD distro but then being off the hook with regard to the rest. That made it possible to freely distribute the BSD distro. The BSD distro existed long before the lawsuit, but you originally had to be a Unix licensee to get it. Then I think Berkeley tried to get rid of the AT&T files and release the rest under BSD4 but there was still some FUD. They got sued and in the settlement they agreed to delete a few more files, which removed any remaining legal clouds.
Fwiw the legal doubts about BSD during that period (pre-settlement) are basically why the Linux kernel became popular despite being far less mature than the BSD kernel at the time. People were afraid to run BSD because of the potential for AT&T lawsuits. The basic Unix userspace utilities were presumably long gone since they were full of AT&T code, but the GNU counterparts mostly existed by then.
I don't think the specifics matter much by now, but I didn't like the misstated history that I responded to.
In the broad strokes, the inaccuracy is to suggest that nobody besides the FSF and RMS were converging to many of the same conclusions at the same time. The FSF did a good job of tying their ideas to a ratchet (GPL and copyright assignment) that would continue to pull in influence. That influence and recognition did not bring any benefit to open source (one of their childish "can't words"). Instead, it merely brought donations into the FSF and starved oxygen from a generation of others.
Regarding BSD itself, there was a lawsuit between AT&T (or some successor) and UC, that was settled by UC having to delete some files from the BSD distro but then being off the hook with regard to the rest. That made it possible to freely distribute the BSD distro. The BSD distro existed long before the lawsuit, but you originally had to be a Unix licensee to get it. Then I think Berkeley tried to get rid of the AT&T files and release the rest under BSD4 but there was still some FUD. They got sued and in the settlement they agreed to delete a few more files, which removed any remaining legal clouds.
Fwiw the legal doubts about BSD during that period (pre-settlement) are basically why the Linux kernel became popular despite being far less mature than the BSD kernel at the time. People were afraid to run BSD because of the potential for AT&T lawsuits. The basic Unix userspace utilities were presumably long gone since they were full of AT&T code, but the GNU counterparts mostly existed by then.
I don't think the specifics matter much by now, but I didn't like the misstated history that I responded to.