Little more than a month after the “Legend of Korra” season three finale aired online, Nickelodeon rolled out “Book Four: Balance.” Fans everywhere rejoiced and despaired. We were still processing the last book’s emotional turmoil, and we needed time to recover. It’s early in the season, but “Legend of Korra” has already hit us with numerous twists and turns. Yet, there are still some things we can bank on not happening.