From what I remember reading, Peter Jackson is helping make another Halo game. Not Halo 4 but just another game.
I have not seen the whole story of Halo 3, just the ending. The ending was good, IMO, though some people say that Halo was just the prequel to Marathon, especially because of the way Halo 3 ended.
All in all, I believe all three games were good and are Xbox Classics. It was just over hyped and that was why many people were disapointed by it.