Don't get too excited just yet. While you will be getting some return for your hard work in multiplayer, don't expect to be getting rich on MSP playing an astronomical amount of Halo 4.
In fact, what you'll be getting for playing multiplayer is a mere pittance: 35 hours for 100 MSP ($1.25), and 140 hours for a whopping 600 MSP ($7.50). Yes, you'll be getting a mere five cents an hour for your time.
Then again, at least Microsoft is incentivizing playing more Halo 4. You could be getting nothing out of your time playing Halo 4.