Go back and listen to the commentary at the end of Part III, right as Michael dies and falls off the chair and the credits roll. FFC talks about the idea of a Part IV and then mentions that Puzo's death is pretty much what stopped it.

I think the idea of showing Vincent controlling the family with young Sonny flashbacks is a good way to go. Probably the best. The problem is, is that The Godfather is about Michael Corleone and Michael wouldn't have much of a role, if any, in a Part IV.

And who wouldn't want Leonard DiCaprio in a Part IV? He's one of the best young(er) actors out there today.

Oh and if there is a Part IV, you've got to find a way to put Benecio Del Toro in it too. That guy's great and has a face for the Godfather movies if I ever saw one.