If she shoots Four, she could save thousands of lives. Every minute she hesitates, people are out there being slaughtered. And yet we get this charming line:
I am sure that nothing is worth killing him for.
Let me analyze that. If he kills her, then her brother and all her friends will die and/or continue on their brainwashed rampage. Thousands of people will die. THOUSANDS. If she's so selfless, she'd just shoot her boyfriend and save the world. If she really thinks Four is so selfless, then she'd believe that he would rather die than let thousands of people die because of him, right?
THOUSANDS OF LIVES ARE AT STAKE. DO YOU UNDERSTAND, TRIS?! ONE LIFE IS NOT WORTH THIS!