I don't think FFC would ever want to work on a Godfather story without Mario. To me it's 100% sure part IV is out of the question. Even if Paramount want it too happen, no actor/writer/diretor would want to participate. Even so, if they wanted too, Paramount f*cked up themselve in the mid-nineties being a pain in the Coppola-Puzo ass when negotiating a part IV (Puzo was writing the script in the early nineties, so it could have been filmed already by then)