I think certainly that G4 was a possibility up to the late 90s, but I think any chance of seeing it now is positively nill, (considering Coppola seems to not have directed a film in nearly ten years, and has shelved his epic, Megaolopis because studios don't like his ideas). And having Puzo - one of his chief collaboraters for the Trilogy - dead also might certainly rule a G4 out. (Plus on the DVD commentary on the credits for GF3, Coppola certainly reveals his reluctance to persue such a project without Puzo).

However, I think one day we may see it as television mini-series most likely without involvement from the films' chief creative people, or a bizarre effort like The Godfather Returns novel - a strange mixutre of both the novels and movies.