I’ve written a couple of times about the recent discovery of the only known Unix v4 tape. The v4 release, which was generally considered lost, is important because it is the first Unix that was written in C and therefore portable.
There are no v4 releases in operation today except, perhaps, for the few instances brought up on a PDP-11 emulator by hobbyists so you might think it doesn’t matter at all to the modern engineer but that would be wrong. Every software engineer should have a basic understanding of operating systems but modern systems are so large and complicated that not even the people who work on them understand the entire thing.
The early Unix systems, on the other hand, were developed mostly by two people who certainly did understand the whole thing as did their colleagues. The advantage of those early systems is that they are complete, working, operational systems but small enough to be understood by anyone who wants to make the effort to learn them.
If you want to learn the basics of operating systems, Unix v4 is an excellent place to start. To make things even easier, Briam Rodriguez has produced a line-by-line commentary on the v4 kernel that is reminiscent of John Lions famous v6 commentary. The repository has the source so that you can build your own version but there’s also a PDF version that you can download and read.
I’ve read Lion’s commentary and have skimmed through Rodriguez’s commentary and Rodriguez’s version seems a worthy successor. If you want to understand the basics of operating systems without getting tangled in the weeds, this is an excellent way of doing so.