Veronica Roth, When Four asks her name, she shortens it to “Tris.” The change reveals that despite her newfound independence, she’s still vulnerable to social pressure. Although she has joined Dauntless because it best represents her individual self, she chooses a name that conforms to the norms of her new faction.
“A new place, a new name. I can be remade here. ‘Tris,’ I say firmly.”
At this moment, not only does Tris adopt her new name, but she adopts a new her. By hating her plain Abnegation name, Tris shows her desire to make a change in her life and start back again. If she had not taken this decision she would have had trouble with the Dauntless. In order to fully prepare herself, she has to completely differentiate herself from the person she was once told to be and from the family that named her. This is yet another of her choices, a choice that changes her identity entirely.