My rating (as a bad movie): 5/5
Wow, just... wow.
The first Phantasm was a classic. No one's quite sure why, because let's face it, the plot was pretty silly, but for some reason, it was just iconic. The second and third were OK, nothing great, but entertaining.
This one was clearly conceived as the magnum opus, where everything would be "explained". Of course that's not what happens...
The first problem is of course to get out of the certain doom at the end of the last movie. If you recall, Mike (still called "boy" in his mid-30s) is in the process of being changed into some sort of hybrid silver ball/alien thingy, and Reggie is pinned to a wall by like a hundred of the killer spheres. Mike apparently just walked away, and the Tall Man inexplicably lets Reggie go - and then spends the rest of the movie trying to kill him. I don't know, maybe he's like a cat playing with its food.
After that, it's just a confusing jumble. The main thread follows Mike as he develops super powers and travels back in time to find the origin of the Tall Man, who we learn was once a kindly undertaker named Jebediah Morningside around the time of the Civil War. He somehow invents the tuning fork/portal thing, but how exactly this turns him into the evil Tall Man is never explained - nor is anything else.
A second thread follows Reggie, who wanders around, encounters a beautiful woman, and is once again sexually frustrated. I know you're saying you've seen that before, but trust me, this time the end is...interesting.
As for Jody, the dead/undead/possible impostor brother, he keeps popping in and out, "guiding" Mike, but never really being much help, and generally just adding to the confusion.
Without giving away any spoilers, I'll just say nothing is ever really resolved or explained.
That said, there is a certain "charm" to the movie. The atmosphere and the cinematography are pretty good, and most scenes alone work pretty well - if only they would all tie together into something even remotely coherent.
All in all, this has to be one of the most random, confusing, non-sequitur movies I've ever seen, and I've seen a lot, but I have to say, it's fun to watch just for the jaw-dropping "WTF!?!" of it.