The animatic stage of production is complete. source
The design/color stage of production is “in the home stretch.” source
Voice recording is likely complete based on the fact that it has been ongoing since at the latest September of last year.
Unclear information / rumors:
Book 4 is rumored to be titled Balance. This is claim is completely unfounded but possible.
Given that Book 1 focused on Korra learning Airbending, Book 2 on the Water Tribes conflict, and Book 3 on the Earth Kindgom, it is likely Book 4 brings us to the Fire Nation and current Fire Lord, Zuko’s daughter.
Kuvira (voiced by Zelda Williams) was name-dropped several times in the finale seemingly without purpose, as well as having a sinister musical theme play when introducing herself. For these reasons it is likely she is being set up for a bigger role in Book 4, possibly as a villain.
Book 4 is rumored to have a January 2015 release based on the fact that this is when the Book 3 Artbook comes out, and the previous Artbooks have had releases timed approximately with the next season. Beyond this grasping-at-straws connection, January seems like a reasonable release time considering how far along production is and the fact that online distribution eliminates Nick’s scheduling issues.
Bryke have stated that 2014 was Korra’s last big SDCC panel, meaning that Book 4 will be released before summer of next year. This further supports the Q1 2015 rumor, as well as the likelihood of us getting our first look at Book 4 at NYCC this year, October 9th-12th.
More and more Book 4 information should be coming out soon, and this post will be kept up-to-date as it happens!
Also, Kuvira appeared in the newspaper that Lin read in episode five, “The Metal Clan”. The Chinese characters under her portrait apparently translate to “Peace Preservation Corps.” (x) Maybe the Peace Preservation Corps. (along with Tenzin and the other airbenders) will take over Korra’s role in restoring peace and balance to the world? Maybe we’ll get a Hydra-esque reveal toward the end of Book 4 (that the Peace Preservation Corps. isn’t all it’s cracked up to be)?