Original: Instead of having each player be an identical Spartan as in previous Halo games, the first player plays as Master Chief, the second player plays as the Arbiter and the other two players control two other Elites; N'tho 'Sraom and Usze 'Taham, each with their own backstories.
Simplified: The first player takes control of Master Chief, the second player plays as the leader of the Elites, Arbiter and the last two as elites themselves.