The OP did specify franchise of 4 films or more. Of sci-fi films, that is. Consequently, that leaves out LOTR ... at least until "The Hobbit" comes out. Once that happens, all best are off.

But the 4 film requirement ... this makes it tougher when naming best and overrated franchises.
For my part, I'd have to say that the
Star Wars is the top franchise considering the wild success of the movies - all inter-trilogy arguments aside. It covers films, books, toys, parodies, pop culture and more. It crosses generations and is still going strong. When it comes to costuming, there is no more fruitful ground for sheer dazzle and beauty in what is created. And the music? None is more iconic than the opening fanfare and the Imperial March.
As for overrated... much as it pains me to say this, I'd have to say "
Star Trek". Its TV/film success has been wildly uneven at best. And this is even in the fanbase. And there is another point. The franchise's longevity I would have to attribute to the fans themselves. There wouldn't be the extended run of TV/films without that fanbase. What has kept Trek in the SFF circles' eye, barely on life-support is the once voluminous number of clubs and websites and fan created activity. IF this next film can be successful, the Trek franchise may live long enough to challenge Star Wars.