The next Book Fire, is when the epic final battle of the war takes place. The Comet that will give the Fire Nation so much power, that no one can beat them will occure in Book Three.
The whole original story will conclude at the end of next season.
What would happen if the show has a 4th season? Michael and Brian original vision is for three season. Will they continue the show, even if it compromises their artest vision? How will the show look?
If there is a 4th season, there will probably be a whole new open credits. The show will be drasticly different, after the epic final battle.
If there is a Book 4, it might be the Jump The Shark moment.