Most of this information is taken from a tumblr news post that has sources, however I will be paraphrasing, but to give credit, this the original source.
Update 1: Added new information
Update 2: Added old information
Update 3: RELEASE DATE!!
Update 4: Added info about Hiroshi Sato per's comment
Update 5: Added new info about Korra/removing disproved speculation.
Update 6: New Clip
Update 7: Trailer
Update 8: Details + Adding Video Section
Book Four Confirmed Information
NOTE: Before continuing I'm going to tell you there are links to videos in a separate; these contain information not mentioned in the Details section
Book four is called balance, , and it will be 13 episodes long and animated by Studio Mir. The new villain has no direct tie to the Red Lotus,, and the final book will air on Friday October 3rd.
- We know that Jason Isaacs is returning, although that was later deleted. However, that doesn't make this tweet any less real, Grey DeLisle's posts about returning to Book 2 and 3 were deleted, even though she did return, and April Stewart's later deleted tweets about joining the Korra cast for Book 2 were true.this
- Per the same tweet, Isaacs's character, Suyin Beifong's VA, Huan's VA and Jinora's VA have a scene together, likely as these characters.
- Korra will definitely have at least a few fight scenes, although I suppose that doesn't really need confirmation. 
- Korra and Katara have at least one scene together 
- The airbenders are working to keep peace as a "Jedi-Knight like role" 
- Per the above source, Book 4 ties into Book 3
- New footage has dropped, as nickelodeon accidentally included Book 4 footage in a trailer about Book 3 airing on television for the first time. In it, Kuvira stands looking very sinister, and in a different part of the trailer looks as if she's about to be attacked. 
- Some animatic sketches from Book 4 here and here.
- Episode 4 is called "The Calling"
- Hiroshi Sato will return
- Korra has a new hair cut. 
- New clip from Book 4! 
- One episode dedicated to Korra's recovery.
- Varrick talks about his childhood in one episode.
None of this information has any official confirmation, but is based on fan theories!
- Since Book 1 was about Korra learning to airbend, Book 2 was about the Water Tribes conflict, and a large amount of Book 3 took place in the Earth Kingdom, fans speculate that Book 4 will take place or have to deal with the Fire Nation. This would match up with Zuko's sudden appearance at Book 3 and suggests he may play a part in Book 4.
- Since Kuvira was name dropped in the finale, and in addition to the new footage, it is possible if not likely she may play a villain in Book 4.
We know a little bit about Book 4. It isn't nothing, but we can begin to start speculating about what's going on, since we do have a few pieces of the puzzle.