Why is it significant that Four nicks Tris's ear during the knife-throwing exercise?

When Tris accuses Four of nicking her ear on purpose, he cryptically retorts, "You know, I’m getting a little tired of waiting for you to catch on!” Although Four does not directly address Tris's complaint, we can deduce his motives from his interactions with her. From the beginning of the initiation program, Tris's unique characteristics quickly draw Four's interest and scrutiny. During a knife-throwing exercise, Tris stands up for Al. Although she does not know it, her actions betray her divergence. It is very likely that Four is first alerted to her unique qualities at this point.
Later, Four’s suspicions are confirmed when he observes Tris’s reactions during a fear simulation practice. Since he knows that Eric hunts divergents, Four also understands the danger Tris is under. In nicking her ear, Four does two things. First, he satisfies Eric's sadistic desire to see Tris punished. Second, he demonstrates his supposed impartiality to Tris and deflects attention away from his growing interest in her. In doing so, Four ensures that Tris is shielded from Eric's scrutiny for some time.