During the trial, Four admits that he joined Dauntless to escape his father's abuse, and that he was going to leave Dauntless to be factionless after this last group of initiates, but he saw Tris and decided to protect her.
That he joined Dauntless to escape his father's abuse, and that he was going to leave Dauntless to be factionless after this last group of initiates. This is what Four admits during the trial. He saw Tris. He decided to protect her.