Four is made up of four – count 'em—four (4) long-ish short stories set in the Divergent world. Three take place Before Divergent (BD), one takes place during Divergent (DD), and then there are three short scenes featuring interactions between Tris and Four, but told from Four's POV.
The first of four Four stories in Four is "The Transfer." Four, then just known as boring ol' Tobias, goes through his aptitude test, just like Tris (then just known as boring ol' Beatrice) did at the beginning of Divergent.
Tattooed Tori tells Tobias that the test tells them… Abnegation. Tobias returns to his home-not-so-sweet-home where he gets smacked around by his abusive father and realizes that he needs to get the heck out of Dodge Abnegation.
At the Choosing Ceremony, Tobias chooses Dauntless, which is surprising to no one who read Divergent or watched the movie (or even just the trailer). When it comes time to jump into the Pit, Tobias, unlike Tris who jumps first, jumps dead last. He goes through his fear simulator where he learns that he only has four fears. They're doozies, but he only has four. Because he desperately wants to leave his old Abnegation life behind, when someone asks him his name he replied that his name is, you got it, Four.
In "The Initiate," Four fights his way to the top of the initiate rankings making friends (Shauna, Zeke) and enemies (Eric) along the way. The initiation is competitive, but nowhere near as dangerous as it will be when Tris joins two years from now.
When he's not smashing faces, Four is getting a tattoo of flames on his ribcage and defying his fear simulation in a way that his trainer, Amar, has never seen before. Amar wonders if Four could be… Divergent! But someone else wonders if Four is Divergent: Divergent's Big Bad, Jeanine Mathews. Jeanine examines Four's data (not as sexy as it sounds) but thanks to Amar's coaching he avoids her suspicion. Unfortunately, Amar doesn't, and ends up dead as a Dauntless doornail.
We move on to "The Son." Whose child is this? Four is Evelyn Eaton's child, but she's been dead for years, right? Wrong. So, so wrong. Four gets a mysterious message from his mother, who is still alive and factionless. She may be homeless, but at least she's the leader of the homeless. She wants Four to join her, but he says, No way, Ma!
She tells him that Max and Jeanine are working together to find a new Dauntless leader, one they can control for their own nefarious means (like monitoring people to see if they're Divergent and taking Pop-Tarts out of all the vending machines). Four isn't anyone's pawn! He's a king—or at least a rook—so he tells Max he doesn't want to be a leader, and Max chooses Eric instead. Tip: Never hire anyone who has more facial piercings than brain cells. This will turn out to be a mistake.
Finally, we have "The Traitor." This story takes place during Divergent. Tris is there, being all Trissy, and Four is wondering what to do with himself. Dauntless is dangerous. Factionless still looks lame. And there's an attack brewing against Abnegation, but does Four care what happens to his dad?
No, he doesn't. But the closer Four gets to Tris (smooch alert!) he realizes that she loves her family, who are Abnegation. Because she loves them, and Four will do anything for her, he decides he'll do what it takes to protect them.