Chances are that the only evidence that the GF Part 4 would be made is a few phone discussions between Puzo and FFC, maybe a face to face or two and some notes scribbled on napkins.

I don't think Part 4 ever got to the script stage. The only way to find out what was planned would be to kidnap FFC and threaten to burn his winery in order to make him confess what he and Mario had planned for a Part IV.

But I do know it involved a plotline similar to Part II with Vincent in the future and young Sonny flashbacks. In that case I'm kind of glad they didn't get around to making it... this trilogy is about the rise and fall of Michael.