Complex sentence: The Dauntless instructor, Four, explains that not all Dauntless initiates will be allowed to enter the faction: only the top ten will stay; the rest will be dismissed and become factionless.
Split into simpler sentences: The Dauntless instructor, Four, explains that not all Dauntless initiates will be allowed to enter the faction. Only the top ten will stay and the rest will be dismissed and become factionless.