For me the story is complete. But I think there are more stories to tell that exist in the Godfather universe. I can't wait to read the new book. That will fuel my interest for a Part IV even more.

I know that a Part IV will probably be worse than Part III (which could mean that it is still pretty good, considering that most movies today are shit) and that would tarnish the legacy even more.

However, any chance to get excited about going back into this world is an offer I can't refuse. I'd see a Part IV no matter how bad it was, who was in it, or who directed it. That's just me. And I could walk away and say that the first 3 films are still great.