The legend of Korra is bad as itself, even when not comparing it to ATLA.
Throughout the show the main protagonists (Korra and Tenzin) let irrelevant things occupy their time, when the writing on the wall is that fucking doomsday is coming. For example Korra goes intio this idiotic benders arena in season 1, and Tenzin does not do any effort to stop her, and it goes on like that the entire show. This is purely infantile, with no regard to ATLA.
The show severely lacks on "true to character". The characters do not have a personality that goes with them - instead their personality changes according what the awful scriptwriters thought would fit each episode.
This is VERY bad. It means the show is a collection of poorly tied together episodes.
This is bad no matter if ATLA ever existed.
The attempt to appeal to pubescent audience (thinking that the kids that were the audience of ATLA grew up) forced in poorly written romance. This also is bad-in-itself, without comparing to ATLA.
Then, Considering it's ATLA sequel, it's even worse. The sequel to a show that had brilliant script, was coherent from start to finish and is counted amongst the best shows ever, so TLoK is awful when compared to ATLA, not worth watching even at the pinch.
The only good thing that came out of TLoK is lewd Korra
Where FET fits in this? cbob, a brilliant scriptwriter, took the poorly writen plot of TLoK and sifted only the elements on which he could build a coherent and even heart-moving plot. The love route beginning especially excels on that.
You don't need to watch TLoK to play FET book 4. In fact, you will be better of without.