In my opinion, a GF IV could only happen if it had Garcia as the Don of the Corleone Family long after Michael's death (last time I saw, Garcia has aged somewhat and would be perfect now) while at the same time, there should be flashbacks to the time bewteen Mary's death and Michael's (Pacino is now looking old enough to be younger than when he died, but older enough as to not have to wear makeup). A flashback to Vito building his empire is a fantastic storyline, but due to practicality it would be a disaster, I think.

Mick