I think if you are going to do a GF4.....you need to make sure that you explain what happened from after Mary's death till Michaels death....Vincents reign of the family, and so on.
That would be hard to do with everyone being soo old now. What makes the GF movies soo great is the settings.....back in the 50's, 60's...the era probably could not be recreated. It would not work in the 2000 era.
I say go back and show us Sonny, Fredo and Michael as kids, experiencing life as a Corleone......show us how they interacted, Sonny's temper, Fredo's wussiness, and Michael defiance. Show us what it was like when Vito was the king.....of course, it would be hard to portray him, but they have done more than that with movies, so it could be done. Maybe have Michael on that chair at the end of GF3, sitting back thinking of his brothers, and their time together.....and maybe when he thinks of Fredo, and killing him.....that is when he drops dead.
Whatcha think???