If you are asking this question you are not alone. So I dug through the Divergent books and found the definitive answer about Tobias Eaton.
Is Four Divergent?
No, Four is not divergent. In the books Four initially appears to be Divergent since he resists the mind control serum and can remain aware during simulations, but he is actually genetically damaged and therefore is not Divergent. We are introduced to this revelation in the third book of the Divergent series, Allegiant.
Here it the quote from the book:
He touches the screen again, and the screen changes, but it remains just as confusing, a web of lines, tangled threads of numbers.
“This is the map of Tobias’s genes,” Matthew says. “As you can see, he has the right genetic components for simulation awareness, but he doesn’t have the same ‘healed’ genes that Tris does.”
My throat is dry, and I feel like I’ve been given bad news, but I still haven’t entirely grasped what that bad news is.
“What does that mean?” I ask.
“It means,” Matthew says, “that you are not Divergent. Your genes are still damaged, but you have a genetic anomaly that allows you to be aware during simulations anyway. You have, in other words, the appearance of a Divergent without actually being one.”
I process the information slowly, piece by piece. I’m not Divergent. I’m not like Tris.
I’m genetically damaged.
Top Reasons Readers Thought He Was Like Tris
In the early books we simply didn’t know any better. Plus Veronica Roth kept that secret hidden from us all (while playing up the possibility that he was divergent).
The tattoos on his back
These were the initial biggest suggestion that Four might be divergent. Plus the much quoted line from Four:
[quote]I don’t want to be just one thing. I can’t. I want to be brave and I want to be selfless, intelligent, and honest and kind.[/quote]
His ability to resist the serum
When Tris wakes up in the Dauntless dorm room and sees everyone moving robotically she does what any smart person would do. Blend in.
Luckily Four blends in as well. Tris believes he was drugged too until on the train car he squeezes her hand and mouths to her “run.” So this was really strong evidence that Four was divergent, just like Tris.
His ability to manipulate simulations
Tobias Eaton got his famous name and identity after he completed a critical part of the Dauntless initiation.
Most people have 10-15 fears but Tobias only had 4, thus his name. His fears are heights, shooting someone, his father Marcus Eaton, and claustrophobia, because his father used to lock him in a closet as punishment.
Anyways, it is during Tris’s simulation that Four discovers Tris’s ability to manipulate the Dauntless simulation. And the only way he could know that she was manipulating it was because he himself must be able to do the same thing.
So now you know the answer to the question of whether or not Four is divergent!