OMGOSH just found Bryan Konietzko's tumblR and he openly states on a topic about the recent leaks of episodes 4-6 that they're currently working on a Book 4!!
"We typically deliver the episodes to the network quite close to their U.S. premieres, but in the case of Book 3, we have actually already finished the entire season as of last week. The network has some plans in the works for it and needs to wait on its release, so we have been keeping quiet while keeping our noses to the Book 4 grindstone. Unfortunately, the longer the episodes sit on all these shelves all over the world, the probability of leaks increases." - Bryan Konietzko