We have touched on this before in another thread. There is actually plenty of scope for a forth Godfather film. As FFC says in the commentry at the end of GF3, he and Puzo had spoken about the 4th film and had a few ideas.

There were 17 years between the opera scene in GF3 and Michaels death. Plenty of time for a movie there. And the movie would also feature the period between GF2 (Old bits) and GF1.

I suggest you read my transcript of FFC's commentry here.