Both Korra and her titular TV show have been looking for purpose in the previous three seasons of the show. Themes are picked up and put down, plot lines changed for unclear reasons that can’t even really be called twists (Unalaq’s layers of villainy for example).
Book 3 introduced a new antagonist, Zaheer, who is uncharacteristically self-balanced. While he uses deceit, he’s presented as single-minded, cruel but also unconflicted. Korra almost fails to stop him and only succeeds with the aid of the new Air Nomads and the metal-benders led Suyin Beifong. Briefly introduced near the end of Book 3 is Suyin’s head of security, Kuvira.
The aftermath of her fight with Zaheer leaves Korra seriously hurt. Book Four skips ahead three years. Korra, having initially gone home to the Southern Water Tribe to recover has gone missing. In reality, she has abandoned her role as Avatar and is making a meagre living fighting cage matches (and losing), while struggling with PTSD, flashbacks to her final fight with Zaheer and the remnants of the poison she was inflicted with by Red Lotus.
With Korra effectively out of the main storyline initially, the show manages to set up a more complex political situation. It has been trying in each season to create a fantasy story with a modernist (i.e. industrial/post-industrial) setting with a realistic political conflict. Both Book 1 and Book 2 relied on villains whose apparent politics were simply covered for a more basic super-villain motivation. Book 3 had Zaheer’s anarchic extremism but aside from toppling all rulers and some sort of inner balance, Zaheer’s ideology was both unclear and also not intended to reflect a mass movement. Book 4 finally manages something more plausible.
After Zaheer in Book 3 murders the Earth Queen, the largest kingdom in the world of Avatar falls into chaos (implied rather than shown). The metal bender Kuvira emerges as a classic military ‘strongman’ style political leader for the Earth Kingdom. Making use of superior technology and a decisive approach, Kuvira is a genuinely nuanced attempt to portray a species of fascism. At no point, until her own plans spiral out of control, is Kuvira portrayed as ‘mad’ or motivated by a dark secret. Like Zaheer, she uses deception pragmatically but her hidden agendas are largely clear. She succeeds in obscuring her main objective (uniting the Earth Kingdom and making it a hegemonic power) primarily by the people around her (and Republic City) deceiving themselves.
Bolin, in particular, is positioned as a sympathiser for Kuvira, working for her directly and actively supporting her cause. Bolin’s role has varied in the series between comic relief and as an ‘everyman’ character. Having him work for Kuvira neatly serves as a way of establishing how she garners support and how people fool themselves about what she is becoming.
The dual track of supporting characters shifting into positions of opposition to Kuvira and Korra’s attempts to regain her role of Avatar give this final season a more interesting structure than the previous three. While Kuvira’s ambitions eventually do push her into a more overt super-villain scheme (a giant mecha to crush Republic City and spirit-based superweapon), the steps on the path don’t require any sudden reveals or psychic manipulation. Kuvira wants to control and Korra seeks balance, eventually enlisting the help of Zaheer to better understand herself.
An unfortunate clip-episode aside, the final half of the season pulls all the strands together into an epic fight that resolves most of the hanging threads of all the characters.
The ending has been much remarked upon. The love triangle between Korra & Mako and Mako & Asami is largely ignored in this season. The dynamic between Korra and Asami prior to the final episode had largely been either in terms of Mako or in terms of them all being part of ‘Team Avatar’ (although at one point in Book 4 Asami does reveal she was corresponding with Korra during some of the time Korra was ‘missing’). What is shown, is consistent with this: Korra and Asami are two people who know each other and have been through a lot together and decide to take a holiday together. The possible romantic connection is about as understated as it can be — partly out of deniability on Nickelodeon’s part but also because the show hasn’t really ever focused on Korra & Asami in any terms other than Mako. Looking back is it more than tokenism? Yes, I think so. Essentially having the two of them metaphorically walk off together into the sunset (or rather accidental new spirit portal created by a superweapon) resolves both a plot line from Book One and states that a romantic relationship between the two characters is a possibility. It is still pretty weak as an example of representing relationships other than heterosexual ones (even more so now that Princess Bubblegum and Marcelline get to kiss in the final episode of Adventure Time) but it is still an important one.