If that were the case, and I really hope it's not, I'd just assume see them re-make the original two movies as a mini-series first.

This way we could get used to other actors playing the Corleones before we see them in the 1960's, in between the events of parts 2 and 3. Because if they just re-cast it and call it The Godfather Part IV, there will be no continuity.

Best case scenario - They don't make it at all.