With Book 3 finally out of the way, we only have Book 4 to look forward to now. With that in mind, I'm making a new master entry that'll pool together everything we know about it so far...
WARNING: SPOILERS AHEAD FOR THOSE WHO HAVEN'T SEEN ALL OF BOOK 3 YET!
- Like Book 3, Book 4 will be 13 episodes long.
- Book 4 will be the last season for both the show and the franchise. (because Bryke wants it that way)
- There will be a new villain on Book 4 for Korra to face, but it will tie in to what happened in Book 3.
- The airbenders temporarily replacing Korra will be a Jedi-like peacekeeping force helping people across the world.
- There's a one-on-one scene between Korra and Katara, apparently over something important. (Janet Varney confirmed that she recorded a scene with Katara's VA)
- Jason Isaacs (Zhao), Jason Marsden (Huan), and Anne Heche (Suyin) play characters in at least one scene with Jinora.
- All episodes will be animated by Studio Mir.
- The animatic stage of production is complete, with the design/color stage "in the home stretch."
- Voice recording is likely complete, save for some possible ADR.
- Janet Varney recorded some fight scenes, implying that Korra recovers enough from her poisoning to fight again.
- There's reportedly a scene in one of the first episodes that's so emotional, it made an animator at Studio Mir cry.
- The rumored title of Book 4 at the moment is "Balance."
- This season is likely set place mostly in the Fire Nation, judging how the books so far have followed the elemental cycle. (Book 1 had Korra learn airbending, Book 2 had a civil war between the Water Tribes, and Book 3 went all across the Earth Kingdom)
- Kuvira, a character voiced by Zelda Williams who was seen in the Book 3 finale, might have a bigger role this season, along with something known as the Peace Preservation Corps. (as hinted in a newspaper article spotted in Book 3)
- David Faustino somewhat hinted that there could be a revival of Makorra.
- There's a chance that info on Book 4 could be revealed at NYCC this year, given how Bryke said they had their final SDCC panel.
- Rumors say that Book 4 could air during or around January 2015. (just in time for the franchise's 10th anniversary in February!)
- Mako and Bolin are accepted into a new and loving family, possibly becoming adopted. (likely untrue now that they have a big family)
- Asami stays single by the end, but Future Industries prospers once more and she becomes rich again. (plus, she could get a man later in life like Toph did...)
- There's supposedly a glimpse into the future of Tenzin's kids in their teen years, being crime fighters of some kind. (possible now that we know the airbenders are Korra's temporary replacement while she heals)