I'm not a great fan of a GF4 but if there was one IMO itshoul go like this,

A don from Chicago minuplates Santino Corelone Jr. and Frank Corleone into thinking that they are the truthful Don. They set up a buissness in Chicago and Vinnie hears it. This Don from Chicago then gets them thinking Anthony is on Vinniees side. Athony is in Sicily visiting Mike in 1997. We see MIke die, the scene from GF3. Tony runs over and shouts Calo who rushes over, they take Mike inside and then we see MIke's funeral. As Anthony leaves with Calo, Sonny Jr. pulls up. He warns them Frank has become engraged at Tony for not joining the family and that Calo should leave Tony alone. Sonny Jr. takes Calo in his car and stabs him. Back in NYC Vincent is married to a girl and has 3 kids. He owns the Barzinis old terotory and lives in there compound. Barzins grandchildren have no idea about their granfather untill Frank tells them. Frank tells Al Neri this barzini relative is a good enforcer and should join the family. This he does and on Marys birthday he kills Vincents consigleri. Over in Sicily Tony is in hiding, only Sonny Jr, knows where he is. He tells Frank and he flies over to Sicily and attemps to kill Tony. He plants a bomb in Tonys house but he is not there. When Tony returns to his home Sonny Jr. turns on Frak and tells Tony. Sonny Jr. has become powermad so he has him killed(similar to Fredo and Mike.) Sonny Jr. now asks Vinnie for help against the Chicago Don. Vincent meets with him and is held hostage, Sonny JR., Al Neri, Connie, Michael Francis and Victor Rizzi assemble an army and attack Chicago. Victor and Tessio's grandchild(a button) are killed. Michael Francis is blinded as Connie sits at a motel she is killed by the bartender. Al Neri dies of a heart attack when he learns of the Corleones demise. Sonny Jr kills the Chicago Don at Chicago airport. Sonny Jr. is in the cockpit with the pilot dressed as the co-pilot. Just as the plane is to take off Sonny Jr. shoots the don. Meanwhile Tony visits Connie and finds her dead he extorts the bartender to telling him were the family is. Tony kills the bartender and takes his gun and gets to the Chicago compound. Tony sees the family dead and assumes that Vincent is dead and that the chicago don is in there, so he blows the place with Vincent still inside. Sonny Jr. is still powermad so when he sees the compound burning, he visions an oppurtunity and does not attempt to save his half-brother. It is an all out battle and the other dons will not get involved. When the consigliri of Vincent is found Barzinis grandchild runs away. He runs to Las Vegas where he sleeps at a motel owned by Eric Segal. The son of Jles Segal & Lucy Mancini...