The film, if it were ever made (and I don't think it should), should not under any circumstances be called "The Godfather Part IV". Remember how FFC wanted Part III to be called "The Death of Michael Corleone" to set it apart from the first two films? With hindsight, I believe that FFC was totally right. Part III was different, but the audiences expected more of the same, resulting in (unjustified) disappoinment. It is hard to see any 'Part IV' being in the same league as the trilogy, so perhaps for that reason alone, it should have another name. It can be a spin-off (that is usually negative, but it needn't be in this case) from the original, developing some of the main characters. How about, "The Rise of Sonny Corleone"?