(Some major spoilers--read at your own risk.)
Apparently Don wrote the script for this in his sleep and started filming the next day without checking for a storyline or inconsistencies. While PHANTASM II was just okay, and PHANTASM III was actually pretty decent, this is definitely the worst sequel. There are so many problems in this film I hardly know where to start, but I'll try.
1. Half of PHANTASM IV is made up of cut scenes from the original PHANTASM. While it's interesting to see the deleted scenes, most of them had no relation to what was happening in IV and did nothing to further the plot or the series in general. Examples: The bit with Mike chasing the ice cream truck, and the part with him visiting Jody's grave. The only deleted scene that had anything to do with IV was the hanging scene (and I can see why they cut it from the original... it wasn't that great).
2. Mike and Reggie were only together for literally about 60 seconds of the movie. These actors have such great chemistry, it's a shame to see them apart for the entire movie. This was probably the biggest reason I did not enjoy the movie.
3. I hate to say it, but Reggie was next to useless in this sequel. The only thing he did that had any real bearing on the story was when he gave Mike the tuning fork. Otherwise about 90% of his screentime was totally useless, especially the entire sequence with the woman and scene with the undead police officers. The one liners were stupid, too. "Blow me"?? Since when did Reggie become Arnold Schwarzenegger?
4. We learned nothing new in this movie except a little bit of the Tall Man's history. There are so many interesting mysteries and unanswered questions from the first three films, surely they could have picked a few points to expound upon? Instead we get a lot of futile running around by the main characters.
5. The time travel and dimension-jumping were done very poorly, resulting in tons of inconsistencies and scenes that were so confusing they didn't seem to make any sense at all. For example, how did Mike know how to use the old-fashioned wheel-and-cog portal apparatus if he'd never seen it before? In the scene where Jody and Mike go back in time to visit the Tall Man, Jody tells Mike he can't kill the Tall Man because they're in different dimensions (Mike tries and he goes right through him like a ghost). But then when the Tall Man returns to the room through the portal, he can see Mike? At one point Jody is seriously maimed and apparently dies, but then in the very next scene he shows up looking good as new, and has to be killed AGAIN. Mike has a dream that he is in a Civil War medical tent being operated on by the Tall Man. It was never explained what signifigance this had (other than Mike fears/hates the Tall Man, which we already know!). Those are just a few of the things that make little sense. I could go on and on.
6. The ending is really lame and the most pathetic excuse for a sequel I have ever seen. As someone else noted, it seems like they just ran out of money and had to stop filming regardless of whether the end made any sense or not. I have no problem with ambiguous or cliff-hanger endings, but by the end of a film SOME element of the plot should be resolved. The Evil Dead movies all have open endings, but there is also a lot of plot development and resolution of problems. There is none of that in PHANTASM IV, and it makes you feel like you watched the whole movie for nothing.
I can only recommend this movie for die-hard fans of the series. If you are interested in seeing the deleted scenes, the little bit of the Tall Man's past, and just watching Reggie and Mike again, then by all means rent it. Just don't expect any plot or character development. If you're looking for a good horror movie, rent the first one!