I personally don't dislike the idea of making a fourth part, because as the Godfather wikipedia says, Coppola wanted to conclude it with Vincent being hunted down and killed by the police. In fact, it would be interesting to see the law enforcement finally DO SOMETHING about the mafia, in the 3 films that have been made they are just almost non-existent dummies.

Willie Marfeo to Henry Tameleo:

1) "You people want a loaf of bread and you throw the crumbs back. Well, fuck you. I ain't closing down."

2) "Get out of here, old man. Go tell Raymond to go shit in his hat. We're not giving you anything."