I am afraid—I am afraid of what I will be asked to do in Dauntless, of what I will want to do. Afraid of some kind of hidden violence inside of me, wrought by my father and by the years of silence my faction forced on me. (1.297)
This is something Tobias/Four has in common with his father—a violence inside. Unlike Marcus, however, Four can usually control himself.
Amar steps back, and Eric rushes forward, fast, his fist hitting me square in the jaw. (2.140)
Eric plays dirty. He's not smart enough to one-up Four in a verbal or mental game, so he has to take physical cheap shots in order to get the upper hand. (And he ends up losing this fight anyway.)
I feel my pain disappear into rage, and I grab his arm to hold him in place as I swing at him again, and again, and again. I don't even see where I'm hitting him; I don't see or hear anything. I am empty, alone, nothing. (2.149)
Here's the violence inside coming out of Four in an uncontrollable flurry of punches. At least it's directed toward an enemy, and not toward a loved one. Does that excuse this outburst?
I hit [Drew] once, twice, three times in the face, and he collapses to the ground, and then I'm kicking him and I can't think, can't think at all. (4.308)
This is a repeat of the rage attack against Eric. Here, however, it's different because Four is attacking someone who threatened Tris, not someone who threatened him personally.
"I'm the one who's threatening you," Shauna says, almost in a growl. "One more violent outburst and I'm going to teach you a lesson about justice." She holds the knife point over his eye, and brings it down slowly, pressing the point to his eyelid. […] "An eye for an eye. A bruise for a bruise." (4.358)
Even the so-called good guys can seem pretty scary. This is the Dauntless way of dealing with problems.