It depends on several factors, including the degree to which the sequels share tropes with the original, the length of the main article, and Wiki Magic. There's no one right way to do it. You should be consistent within a franchise, though.
edited 6th Feb '11 8:34:06 AM by Fighteer
"It's Occam's Shuriken! If the answer is elusive, never rule out ninjas!"Pretty much what Fighteer said. There Is No Such Thing As Notability means that any work is relevant enough to get a page, even fanfiction. I doubt that Fantastic Four Rise Of The Silver Surfer is genuinely popular enough to be seperate from Fantastic Four, but if someone wanted to there isn't any reason not to.
How that page is organized within the wiki is really what matters. For example some franchises have an absolutely massive Expanded Universe, so instead of placing all the EU stories on the Star Wars main page, the index of those EU stories go on the Star Wars Expanded Universe page. Also, the Tim Burton era Batman films use the movie Batman to outline the 1989 film while giving directions where to find Batman Returns, Batman Forever and Batman And Robin.

Some sequel entries seem to have their own pages instead of being part of the franchise's page.