To expand on Hollywood Hagen's point, there should be no Godfather film without Al Pacino as Michael Corleone. The trilogy is about Michael, the central focus should always be on Michael, it tells us of how his decision and actions as Don Corleone ruined his life and effected the people he loved.

And that story has been told. Michael has paid for his past sins with the death of his daughter, and has lived out his life agonising over his past failures. Why Part IV? All Part IV would do is "fill in the blanks" if you will, of the Family's rise to power in the underworld in the 30s, and how Vincent brings The Family to ruins in the 90s. Is this necessary? The Trilogy seems to work fine without the material we will see in a Part IV.

But hey, it's great fun knocking about Part IV ideas here on the board, (hell I'm writing one now!), but there really should be no Part IV without Mario Puzo. With his vision we got three great movies, which we'll treasure for years to come. And Michael's story, through the trilogy, has been told, and as I've stressed before, his character and story-arc are complete.