4.1 is Left, Centre, Right, Monaural surround and subwoofer; essentially the same setup as Dolby Surround, but not matrixed. The original 6-track configuration for the 50s and 60s was Left, Centre Left, Centre, Centre Right, Right, and Monaural surround, with no subwoofer track; this was modified in the late 70s and early 80s to the first configuration described, with the Centre Left and Centre Right channels becoming "baby booms" or subwoofer tracks.
The changes attributed to the 70mm mix have something to do with different sound effects and music cues. I don't have the specifics, but they are buried in one of the older threads in either this forum or the General Star Wars Discussion forum. To access them, you'd have to customise the forum. Hope this helps.