Why Some Sequels Thrive and Others Flop

This seems like a very canny way of making more money in Hollywood.

"Continuity is certainly a plus when it comes to series," said Brandon
Grey, founder of Box Office Mojo. "Sequels should have a similar feel as their
predecessors and should be in the same universe."

Director Peter Jackson understood that when he shot all three "Lord of
the Rings" films at the same time, ensuring that the same cast, the same
director, the same special effects guys and even the same grips were involved in
each film. There was no quibbling from the audience over who made a better
Legolas; it was the same fresh-faced Orlando Bloom each time.

If they love these some actors in these same roles in the first place they will love them again if the story is as compelling as the first three. People are willing to make movies into $300 million domestic blockbusters only if they get more of the same but just slightly different. They want to feel the same way they felt the first time around as long as things are freshened up a bit.

Spiderman 3 was pretty much the continuation of the love story in the first 2 but with different villians and challenges faced by the main character. I think keeping everyone the same and introducing new characters allows the audience to lose themselves in the same world that they has already made the studio $300 million just a few years ago. So now we have Harry Osborn out but Gwen and Capatain Stacy in. Spiderman 4 may not make more then the first three but I would be willing to bet even money that it will break the $300 million mark easily.