This may seem silly or spurious, and, well, it is.
I found AT&T UNIX System V disks without the C compiler system, and wanted a working compiler. So I made one. Out of pcc. The attached patches add a working i386-svr4 target to pcc 1.0.0, and pcc-libs 1.0.0.
Only posting this on the issue tracker for posterity. If you feel it's too absurd of a platform to add as a target, leave it out :)
The target files are based on sunos, since the two OSes are basically the same. In pcc-libs I actually just extended the sunos target to svr4*|sunos*|solaris*, since they're literally identical for those purposes.
I cannot confirm that any of this works with the original headers or crt files since I had to cobble those together myself, plus the backend depends on GNU ld since I don't have the original linker (it uses GNU-specific --dynamic-linker flag). But I can confirm that this runs on a real AT&T UNIX System V/386 Release 4.0 system, and produces working binaries.