Forgive me if I repeat anything, but I haven't been on this page for months (no-one will remember me probably), but if part four is a possibility, then is it not fairly straightforward the most obvious way to work it would be Garcia as an older vincent, de niro to play an older vito circa wars with the irish, pacino to play the reluctant consigliere post marys death, and a newbie to play the young sonny in the vito days. I know the lack of brando is off putting (he is the godfather), but i can see no other way of playing it. The flash backs of GF2 would be an appropriate structure for development. Again apologies if this is a fairly common thought but i had to post my thoughts.

i think getting those actors would be difficult. i would write the script after the actors have agreed to do it. if pacino, de niro, and garcia all agree to do it...then the plot should be like you said. i doubt pacino would do it, and i know garcia would. so michael will probably not be vincent's counselor....also getting to de niro to play an older vito....might be difficult. if he's not gotten....it's going to stricly follow sonny's life probably....him maybe having a small business (part of the family business...father gave it to him to test him).....thus he never has to see his father exept talk on the phone or though messages.

I don't know why you guys stuck to this idea of flashbacks...

maybe it would be good, specially with De Niro playing Vito, but there are many other possibilities. (I hope no one to be made and I think no one actually will)

I believe Pacino and FFC would not agree to make GF4. Pacino is just a guess, but we know FFC seems regretful about GF3 (I saw him on actor's studio and he didn't look happy about having made part III)

Also, I think (and I hope all of you agree with me, please!!!) a godfather movie without Al Pacino would be ABSOLUTELY ridiculous

I can't agree with you. I think that gf4 could be as good as first two whitout Al Pacino,
but Andy Garcia should be in the movie.

well, you can think whatever you want.

I just can't even think about The Godfather without Al Pacino.

i guess it could be as good....but i would really like to see de niro come aboard. garcia and another actor could pull it off....but i somehow also can't see it being as good without pacino. pacino was amazing in all three parts (though vincent was very good in part III)....without pacino i'm scared the quality of acting will drop a great deal, though.

Thinking of a Godfather without Pacino seems unholy..
So then i guess its time to be unholy!

Talia Shire is an excellent actress who really shines when she is surrounded by other great actors. The chemistry between she and Al in the trilogy is one of the great on screen pairings in movies history. Garcia isn't enough to encourage a knock out performance from her though.

Andy Garcia can be both overrated and underrated. Underrated in that he was fabulous if Part III and there are those who forget that and overrated by people who say "oh he's an Oscar nominee' and forget his movies since have bit big time.

All that said, i think it underscores he fact that we would need to do the flashback thing. I truly believe that Deniro would be game (if he agreed to Analyze That he should embrace this with open arms!) And in these flashback scenes PLEASE hire people who will knock your socks off, unknowns or otherwise.
So i would keep:
80's Era:
Talia Shire
Andy Garcia
Bridget Fonda (who CAN act)
Nicholas Cage in some form (if your gonna go to the well, get someone GOOD
Franc D'ambrosio (for an Opera-esque scene)
and even, don't kill me
Diane Lane and Madonna-Diane could be the new Kaye type and Madonna the new Connie. I think it would be a nice Juxtaposition

Sorry, but no. Madonna is dire. In fact she's worse than that.

But I see no reason why Pacino cannot be in it. He died at the end of the third, but the time scale enables him to be in it. He could work with Garcia as a type of consigliere, and you could have him pondering where his fathers empire turned into his monstrosity where he alienated everyone- cue flashbacks about a young sonny, the ageing vito (de niro) and the wars with the irish, or running molasses during prohibition. How vito met luca Brasi etc,how he killed maranzallo (?)and sent Capone packing (stuff from the book). If done correctly this could be pulled off. For credibility i'd like shire, Pacino, De niro, garcia to act, and Coppola to direct.

Oh and the emphasis placed on flashbacks is due to the fact Garcia could not handle the movie on his own. An the film could not be carried solely on the premise of his empire. Perhaps for this angle, we could see the downfall of the corleone empire through vincent becoming involved in narcotics (he's sonnys boy after all)