whats the name of book 4 of avatar the last airbender?

answers1:airwhat is airbender?
answers2:air. The book names fallow the cycle of elements from which
the avatar is born. In the show he starts with air,but then learns
water and so on, so the cycle goes like this: water, earth, fire, air,
water... and so on and that's how you get the names for the books
answers3:sorry, there's no book 4.
http://www.avatarspiritmedia.net/faq.phpIf there was, it would
probably be air but what would be the story? aang learns waterbending
in bk1, earth in bk2, and fire in bk3. He already mastered air.
answers4:I' don't think theres gonna be book 4. Because in book 3 it
will be Aang's final battle. Unless if nickelodeon gives it a twist