Can't fight that. I've been trying to defend Book 4 for awhile now, but I think it's legitimately my least favorite Book in all of Avatar. At the very least I really enjoyed Book 2's action even if it's plot arc was wonky as fuck at times. And I might be the only person here who genuinely enjoyed Unalaq as a villain (I've analyzed him a few times before and why I like him).
While I liked Book 4's action (so much fucking metalbending) and I did enjoy the ending (truly), Book 4 had a lot of missed opportunities that hurt it in my book (hah). I enjoyed pro-bending and I didn't think the romance was all that bad. Not nearly as much as other people make it out to be. Balance's ending tore me up for the longest time, because I had a ton of emotions and no where to put them. I saw the moments, I felt, but I couldn't release because they lacked weight. Air may not carry as strong of an emotional connection, but it at least hits the right beats at the right times and makes them count. That's for me, of course.