Having read most of the 5th Symphony, it doesn't really mesh that well with the end of the fourth, which was pretty conclusive on it's own.

Also, while the Legend of Korra diversion might have been nice to some, it took a segment of the overall saga that was already meandering and took that down another rabbit trail.

I suppose the 5th could be made to fit in better with the fourth with a rewrite, and the LoK segue could still happen, but only after the rest of S5 is rebooted to line up with the preceding canon better.