Giving it to Chief. Spartan IV's trained for the beginning of their careers to be marines, only after being showing excellent combat skills were they augmented to be Spartan IV's. The Chief trained since the beginning of his LIFE to be a Spartan II, and you can say what you want about luck, because even among the Spartan II's he's revered as a legend. The Spartan IV's are, from what I understand of what we've been given so far, are a less effective, cheaper version of Spartan III's. And Spartan III's are a less effective, cheaper version of Spartan II's. And Master Chief is the king of the Spartan II's. The IV's are cool and all, but from what I understand, it's like pitting a fox against the king of the wolves.