IGN exclusive interview with Mike and Bryan, discussing the Book 3 finale and Book 4. The main points of new information learned are:
"The show is essentially done" says Bryan, they are working on post-production for "the rest of the year."
Book 4 will have a new villain that is not a spirit or Red Lotus member.
Bryke confirms the Airbenders will take on a peacekeeping Jedi-like role.
Before the Hundred Year War, the Air Nomads did not mingle with the other Nations, staying detached from society. This is slightly different from the impression we got with Aang saying he had friends all over the world.
Zaheer was a master martial artist and professional fighter before being imprisoned and getting Airbending.
The Korra video game, set between Books 2 and 3, has its own new villain.