Thanks to the fandom's amazing work on Tumblr we have unlocked a clip of Book 4 Balance on Nick.com
http://www.nick.com/videos/clip/korra-215-clip.html
Use Tunnelbear or HolaBetterInternet if you are outside of USA to access this clip
Based on the URL of the clip it seems like this is from the second episode of Book 4, but Nick's numbering is a little odd. Nick also confirmed via twitter that the Book 4 Premiere on October 3rd will be 1 episode, but again plans can change.
Interesting clip in that Korra seems to have decided to return home to the Southern Water Tribe for a few weeks, so I guess at some point there is a time skip where we see Korra get her new Book 4 design.
Thoughts?