I don't think it would be appropriate to have Pacino in part IV, if they make it. It just doesn't make sense for him to be in it.
1) He would be too old to play in the movie and his character as 'Michael' is dead in part III.
2) The part 'Michael' cannot be shown at his younger age cos he didn't rise from a scrappy person in a bad neighbourhood and became a mafia. His character is different than Vito.
I think that if they were to make part IV, they should concentrate on Vincent in the present world and all the other family members. If they want to portray the past, they should focus on Sonny and the brothers and maybe Vito as well.

After all, Vincent is Sonny's boy.