One of the things that I enjoy the most about this show is how each season moves a massive amount of pieces into place that all come together somehow in the end. Peter's campaign for Governor, Cary's plan to escape Lockhart & Gardner and convince Alicia to join him, Alicia struggling to figure what it means to be a partner, redefining her relationship with Peter and trying to regain trust while dealing with her feelings for Will (and everyone else telling her how she feels about Will, while she never articulated any of it), Diane tying her future hope to Peter Florrick - everything is connected in the finale.
Even the way that the show has constantly portrayed Zach Florrick as the character who is so very much his mother's son yet a technologically weaponized version of her is essential in What's in the Box. He is about to vote for the first time in his life, for his dad, and notices that at the polling station, a box of votes with a broken seal is carried in suspiciously. Taking a picture of it with his phone comes naturally to him - he documents, shares, he's never known a world where wasn't as immediate as it is now (and as once before, he's told he CAN'T, but insists, and now he knows his rights as well, knows that taking pictures in a polling place isn't illegal). And because he's Alicia's and Peter's son, the consequences are immediate. He calls Alicia, Alicia calls Eli, Eli asks the firm to take the case while time is running out, because it's the night before the election - and Patti Nyholm represents Kresteva (and the really interesting bit is how everybody just naturally assumes that the Kresteva campaign would be the one tampering the results).
Patti: Do you question authority a lot?Zach: No. Only when it's wrong.
After the witnesses (among them, naturally, Zach, and the elderly Mrs Eisenhower, the REPUBLICAN monitor who reacted so aggressively when Zach voiced his complaints) have been questioned, Alicia and Patti take a little detour when the third party candidate argues that all the ballots are invalid because his name lacks the essential Irish apostrophe - and it turns out that The Third Party Candidate is where one Jordan Karahalios went, and he holds a grudge against Peter's campaign because of the way he was disposed of. Eventually, the box of secrets is opened, the votes are counted, and surprisingly, they are vastly in favour of Peter Florrick (and still, Alicia doesn't put the pieces together) - and Patti and Alicia hilariously switch places, arguing the opposite position, attacking their own previous witnesses (because Eli and Peter think the race will be close, those votes may be essential to guarantee a win). Judge Abernathy, after Jordan gives testimony that Eli was planning to stuff ballot boxes back when they were both working Peter's campaign, rules to exclude the possibly deciding votes.
Meanwhile, Cary Agos and his fellow fourth years are looking at an office space that is beautiful but costs too much, and the others tell him they could afford it as long as they don't have to pay for Kalinda - but Cary knows that Kalinda wins cases, but they know that Cary isn't objective on Kalinda (both of these things are true). They also run into Colin Sweeney, who happens to own the office they have their eyes on and offers them a better deal on the price in exchange for some years of free legal services, during which he presumably could kill a whole string of fiancés and wives - but of course the offer only stands if Alicia is part of that team.
So in summary, that's being able to afford the office space either by losing Kalinda (who asks for a share of their winnings as a compromise, but a beat later we see that Cary has a contingency plan, if he can't get Kalinda, he'll take the one person who picked up on everything Kalinda can do with a freakish speed, and her name is Robyn Burdine) or by striking a deal with the devil aka Colin Sweeney, which only works if Alicia changes her mind about Lockhart & Gardner.
Alicia changing her mind... Alicia spends the episode working close with Will trying to save Peter's bid for Governor, and it drives home the fact that things have changed between them since the kiss. They get the ruling reversed when Zach turns out to be the only person for some reason to actually remember that Eli wasn't in charge of the campaign when Jordan claimed he decided to stuff ballot boxes, and in the end, it turns out to be irrelevant anyway - all of it, the entire case, the Democratic Committee Chair handily offering to cause a septic spillage in Kresteva's best precinct (he didn't even have to think about that one, it came as naturally to him as Zach taking that picture with his phone), because it's a landslide victory for Peter.
BUT. Before the victory is declared but after the suit is decided, Kalinda finds something, and she doesn't take it to Alicia, she takes it to Will, and Will won't take it to Alicia, because both of them know that it would be painful for her. It's a video of someone from the Florrick campaign delivering the box. And remember that Eli told the to pursue the case, assuming that it was Kresteva's plot. Whose decision was this? Did the Democratic Chair intervene without anyone's knowledge?
Will takes it to Peter. He tells him that he didn't take it to Alicia "because he didn't want to hurt her" and asks Peter what to do with the video that would cost him everything (regardless of how the election ends, it would presumably disqualify him). It's always been difficult to read Peter Florrick, so we don't know if Peter knew, Will doesn't know if Peter knew, and the whole thing isn't about doing the morally right thing, it's a weird power play between the two. Peter doesn't own up to it - he tells Will to do whatever he wants with the video, "Want me to lose, show it to the judge. You want me to win, don't. I'm not owning this decision." Will does nothing with it. Peter wins (everything except his fight to remove Cristian from his mother's life, which only gets him a fucking terrifying "It's my life. Not yours. Don't mess with it. Ever." from his mum - and MAYBE IT WAS JACKIE ALL ALONG, who knows). None of them tell Alicia (KALINDA doesn't tell Alicia).
I think she buried all her suspicions, the weirdness of it, the not-quite trusting but really wanting to trust Peter, deeply, but at the same time, she doesn't seem completely surprised when she sees the Republican (how often the all asked him that question) polling monitor at Peter's victory celebration. She REALIZES. It's not even important who was responsible, but how she feels after - the "I need to freshen up" is the polite way of saying it. She looks into a mirror (not sure but in a way she's avoided looking into the mirror this entire season, throughout the process of losing the privilege of walking out of rooms without feeling slightly gross) and makes a choice. She walks out of the celebration - and we don't see it, actually, but it's the perfect imaginary companion shot to how all of this started, Peter alone on stage, without his good wife - makes a phone call, and of course the person knocking isn't Will.
Cary: What are you thinking?
Alicia: I'm in.
(I'm not entirely certain if I will do these for the fifth season but the idea of Alicia working against L&G, against Kalinda and Will, DOES sound pretty awesome)