Solaris was basically SysV.4. Sun/Oracleopen-sourced Solaris, and then made Solaris closed again, but OpenIndiana/Illumos forked a version, so SCO/Novell(?) had to approve that.

I don’t think that necessarily means SysV.4 can be used directly without permission, but it suggests that SCO might be amenable to giving permission. Write a letter and ask.

But if they don’t, everything you need is in the Illumos repositories.

The installation instructions on AAP's squoze site are defective. PiDP uses those defective installation instructions so it's all messed up too. If you want to install it correctly, the instructions are in this GitHub Gist:

https://gist.github.com/ambiamber/7b1f01c1028b9942ac00d90213279b9c

P.S. the first UNIX system I ever used with SysVr2 on a PDP-11/45. Before that I only had work-alikes such as MINIX, etc.

P.P.S the former SCO UNIX, SysVR5 (yes "R5", go look it up) is now owned by Xinuos (isn't that clever--spelling unix backwards like Mt. XINU, et. al.?). Xinuos is still branding it as SCO. Rob Pike wrote that people treat UNIX v8, v9 and v10 as if they do not exist. Guess what? DMR released a usable copy of v8 for VAX-11/780 and it runs on SIMH. Norman released a usable copy of v9 for Sun 3/50 and Sun 3/75. TME Sun 3/160 is compatible. Admittedly someone wrote that the Sun 3 port of v9 is a partial snapshot-like creation and that v9 development continued on the VAX not the Sun. Notably the uniq command is missing from the sources and binaries. I'm not sure if it was intentionally dropped or just an oversight. All the manual pages and the man command and *roff commands are missing too. There might be others. The v8 and/or v10 sources could probably be used as a substitute for the missing v9 software. The v10 artifacts are only source code and documentation, etc., no binaries. There is the slightest chance of being able to use v10 source as an upgrade to v9. There is a PDF of a letter circulating around that is purportedly Nokia/Lucent reporting that they will not prosecute for using v8, v9 and v10. Cat-v is not a cult, it's not even a sketchy religion. Cat-v is the purest UNIX-foo. Come with us and hate BSD, hate sockets, hate commercial "unix" junkware like SysV. Love only pure research unix and plan 9 and its derivatives.

No, none of these are legal to fork off of and even if they were SCO is technically still around and litigious as ever so I wouldn't test my luck. The (genetic) Unix versions which are (likely) free to fork and use are the so-called Ancient Unix versions (V1-V7 and 32/V released under a 4 clause BSD equivalent license in 2002 by Caldera International illegitimately, the current copyright holders at Novell/Micro Focus/OpenText/whatever they are this week said they would honor the license grant and show no interest in pursuing people for using said sources, USL v. BSDi also seems to imply that most of these sources are in the public domain, but again, IANAL and don't hold me to anything around this), 4.4BSD-Lite/4.4BSD-Lite2 and descendants (assuming the prior here about 32V copyright being invalid isn't true, which I have substantial reason to doubt), Plan 9 from Bell Labs (which AT&T held on to, eventually spun out with Bell Labs, and the successors in interest comprehensively freed a few years ago), and illumos (the only System V descendant which is legitimately FOSS, this probably comes closest to what you're asking for assuming you're willing to use CDDL-licensed code).

Is the Unix System V R4 source code legal to fork, I because I want to make my own unix based system.