Part of the whole deal with Godfather 3 is that most of the characters you were accustomed to in the first two films aren't around. Its like a band that's been around for a long time once it gets to the point to where there's only one original member people lose interest. Speaking of which, that's why I think they had that huge ceremony at the very beginning of the film to show as many of the people from the other films as possible. Because they were light on main characters.

Also I don't think Godfather 4 will ever happen. Part 3 was supposed to be a separate title (the death of michael corleone) and not making the films into an actual trilogy.