There's no deal.

All fanfic in here: Some don't want a GF IV, others do.
Some favor no flashback's, so only the story of Vincent/Michael (1980 -1997). Others want flashback's (1927 -1945). Some even want to cover the period 1960 - 1979. And there are also people who want to continue the story from where Michael died.

Fact is, Mario Puzo wrote a part of the script before he died (RIP 1999). Nobody knows what it is about, which timeline, or how far he finished it. I think some stated he was about half.