[TUHS] Unix 4.0?

[Warren Toomey via TUHS]

Strangely, I was just browsing through the Unix Archive here and came
cross https://www.tuhs.org/Archive/Documentation/Manuals/Unix_4.0/

For those looking for a reference guide, there is one here which is a
flip book with six holes for binding at the top. It names itself
"UNIX Reference Guide Release 4.0" and is dated April 1981.

So this isn't 4th Edition :-) Is this release 4.0 of the reference
guide, or does the 4.0 refer the version of Unix. I'm confused!

Cheers, Warren