I just found out that apparently 4 track magnetic audio prints were made up until 1983 ending with Scarface. Is it possible that there were mag-optical combo prints of the OT? There’s a thread over on filmtech where someone remembers running a magnetic/optical ESB print.
It is possible that if Star Wars had magnetic prints that it would indeed be a fourth mix as it had different standards than Dolby Stereo and a limited mono surround controlled by a test tone.