I dont know maybe I'm crazy but the only thing I can see 4 being about is the time from the end of the prequel portion of two up until one. Really that would probably be the best portion of the saga, showing how Vito built The Family up into its glory period. There are just endless possibilities for the plot also. I think that would be as Puzo wanted it too. Isn't this far and away the best possible scenario for 4?