A direct continuation from Part 3, the adult Mike, realizing he may be dead (with a sphere for a brain), takes off in the car, leaving Reggie to venture out on his own. There are lots of flashbacks, which is cool, and I suspect deleted scenes from the very first film. Mike talks to his brother Jody, goes back in time to see how the "Tall Man' came to be (he was an old inventor who bridged the dimensions and came back evil). Reggie meets up with a young blonde woman and she turns out to be a creature created by the Tall Man.
On rewatching, the first three PHANTASM movies hold up quite well and are as good as I remembered. However, this much more surreal fourth chapter still didn't make a lot of sense to me. Oh well.