It doesnt make sense to have a Godfather IV. Anything about the rise of the family during prohibition would make it nothing more than a typical gangster genre movie, which is precisely the mold the original Godfather broke.

A "post Mary's death" story with Andy Garcia at the helm would necessarily involve an impaired Michael, and I dont see Pacino playing that part very well. Besides it would have to show the deaths of all the remaining original characters including Johnny Fontaine. This would just be a big bummer.

The only other option would be to pick up during the time frame of that horrible book The Godfather Returns. I suppose they could do something with the JFK asasination and perhaps something showing how the Corleones began cleaning up their finances by paying off the Nixon administration, but all in all the Godfather franchise ain't broke, so it doesnt need to be fixed.