At it's inception, it was conceived as one book, 12 episodes. Once work got started, Nick liked it so much that they expanded that to 52 episodes that the creators divided into 4 books. That's why book 1 seems a bit more neatly (hastily) wrapped up whereas the rest flow into one another.

Once it began airing (particularly on Friday at 8 rather than Saturday mornings) it turned out that the on-air ratings weren't spectacular but the online views (and piracy) were strong. So Nick pushed it fully digital (rather abruptly) where the second half of book 3 and all of book 4 aired.

Korra had it's full, intended run (moreso even than originally planned), so it was never canceled. There were never formal plans for anything more than book 4.


The disheartening thing for me is that the ratings performance doesn't bode well for future trips into the World of Avatar.