Four runs Tris through the fear landscape again. Unfortunately, while it's just a simulation, the fear is bleeding into her actual nightmares.
This time, Tris's fear is drowning in a glass box while people ignore her.
But instead of drowning, Tris breaks the glass, which is supposed to be impossible.
And that's how Four figures out that Tris is Divergent. But he'll hide the info for her. (Which is proof that he likes her, right? We're not crazy, right?)
Tris goes to chat with Tori about this super secret thing (that quite a few people know about now). Tori tells her that being Divergent means that she knows she's in a simulation and can change things.
And if the Dauntless leaders find out, they'll kill her. So that's not good.
In fact, that's what happened to Tori's brother. They made it look like a suicide-by-chasm, but she's sure it was murder.
So Tris thinks, if Four wanted to help her, either he is Divergent or he knows about Divergents.