Here's the first installment of my basic outline for GF IV, with more to come:

First scene is Michael's funeral in Sicily. All remaining family members are there, including Vincent. Vincent has charges piling up against him in court back in the states, and he's lost now without Michael. The Corleone Family is in disarray. One by one his uncle's old associates were either killed or sent to jail. He reflects on the stories Michael told him over the years about growing up with Vincent's father, Sonny...

(We hear a voice-over of Al Pacino starting his story, and then we cut to...) a young Sonny Corleone, in the 20's. We have the scene where Sonny finds Tom Hagen in the street and brings him home. Vito takes pity on him and adopts him as his own son. A storyline concerning Vito taking care of the people who ruined his newly-adopted son's life ensues.

Cut back to 1997. Vincent is on trial for some mafia-related charges. The trial is begins smoothly, and then his half-brother Santino Jr. shows up. Santino Jr. had always resented Vincent, but now he came to Vincent for his help. He had made some bad deals with the Cubans down in Miami, and now turns to Vincent to bail him out. Vincent doesn't completely trust him, but decides to take his half-brother under his wing and teach him the finer points of the family business.

Cut back to a teenage Sonny, sometime around 1932. The Don has been building up his power slowly, but it soon appears that there will be a full-scale mafia war. Vito decides that it is now time to tell his oldest son about the family business. He explains the essence of the Corleone Family, and starts him out by giving him a small-time job, tailing a powerful member of another family, to try to pick up any information about his associates, hangouts, etc. He tells his father that the man liked to frequent a certain restaurant in the Bronx. The Don thanks and congratulates his son on a good job. The next day, Sonny watches as the man is killed outside this restaurant. He realizes that he helped kill this man, and comes to terms with it. He is now a cold-blooded killer.