I'm starting to think that the merit of this franchise will depend on whether the final installment pays off. The longer it goes the more mysteries it throws out and if the filmmakers don't have an adequate solution to it then it may not be worthwhile. While part 2 and 3 were fun, it becomes a series of cliffhangers and eventually you will want some resolution.
They did get around to Phantasm 5 Ravager. Unless you are a fan of the series(which I am) it probably will not seem all that good. We do get some real clarification and closure if you understand Don Coscarelli doesn't see the world in quite the way most people do.
His movie John Dies In The End is required watching if you like Phantasm. It is based on a horror/comedy novel by David Wong. Ravagers commentary track is pretty good as well. How they worked around having no money or permission to film in some areas is fun. Also Angus Scrim aka the Tall Man got to see the finished movie right before he passed away.