For those complaining or worrying about the pace, I'd like to point out that Season 3 ran at a similar pace until episode 8. Korra never even met Zaheer or was aware that he was a threat until episode 8. Before that the action focused on the Earth Queen, finding and training airbenders and the metal clan. Episode 8 she almost got kidnapped and they found out who was the snitch. Episode 9 she locates and talks to Zaheer and gets kidnapped by the Earth Kingdom. Episode 10 Zaheer kills the queen. Episode 11 Zaheer takes the air nation hostage. Episode 12 Korra gives herself up to Zaheer resulting in a huge battle cause P'Li's death. Episode 13 Final showdown.
The plot accelerated from 0 to 60 fairly quickly. I think there is still plenty of opportunity to have a big grand finale.
I see where you are coming from, but I don't really agree. Basically, I agree that the pacing for the main plot in Book 3 was strange. And in the grand scheme of things, we can even look back and say it was a misstep (since Zaheer needed more time to be fleshed out on a character level by the end of it). But this is why I don't think the comparison is 1:1.
First off, Book 3's side plots were quick paced. And while Zaheer and the main plot (ie. his motivation and Korra and him clashing) wasn't until the back half of the season, Zaheer + his group were a lot of fun to watch. Their jail break scenes, their fight sequences trying to get to Korra. It was all exhilarating stuff. Anytime they were on screen it was just pure entertainment. The air bending nation plot, the earth kingdom stuff, and even Lin and Su's back plot were all interesting stuff (on a smaller scale).
So while I agree that the pacing for the main plot was also kind of strange, I think the actual content that was the first half of Book 3, was either I. Fun. II. Interesting. III. Entertaining. Book 4 is different, in that most of the plot is a singular focus (Kuvira & Korra being broken). These two plot points are the sole focus, and run parallel. I guess it's subjective opinion, but I don't find Kuvira or her movement anywhere as entertaining or exciting as Zaheer, even if we didn't know what Zaheer or his motivations were until much later.
I also don't find the episodes in Book 4 up to this point, as fun/entertaining as Book 3's episodes were (on a self-contained individual level). I think Book 3 was aided, by Korra and the group traveling around (the call back to ATLA's adventure style) and so the side plot and whatever the episode was focused on (while maybe not important, like something like Kai)...were still fun and entertaining.
Kuvira on the other hand, is pretty stoic. She hasn't really played her cards, and is pretty steady with her image. We already know the drill with what her plans are. And it's just kind of a slow burn of her getting bigger and bigger, and Korra being off on her plot feeling broken and down. Book 4 is past the half way point, and everything feels like nothing progressed, and has actually taken steps back. All of Korra's plot, feels like it fell apart and needed to start over when she lost to Kuvira after 4 episodes being dedicated to her training. We finally got to see an interesting side to Kuvira, when she got smug and took delight in beating Korra down in front of others. But outside of that, we really don't know anymore about Kuvira. And again, her taking over the Earth Empire was already basically in motion since Episode 2. Her taking metal city wasn't really a big revelation. I would say the ONLY thing that has progressed with her plot, is the spirit vine stuff.
So yeah, in comparison I just think Book 4 feels a ton slower. We have these two single plot points, and they feel like they are running in place. Whereas Book 3 kind of felt like a mish mash of things. It was basically Team Avatar doing adventure shit, while a looming threat was off in the distance getting bigger and bigger. Book 4 is more like, the threat is already known. We already know what it is. Korra is just broken and not able to stand up to it. So it's pretty much all about Korra getting to a place to finally step up.
If they would have done more with Book 4's political plot, and asking a bit more interesting questions about the Role of the Avatar (and focusing on it from that perspective, rather than Korra just not being confident) -- then I would have said this Season could have been as interesting.