If, like a large proportion of people currently on Xbox Live, you are already playing Halo 4 multiplayer online, then you might be interested in the game's latest Xbox Live Rewards promotion, which awards you Microsoft Points for playing the multiplayer for a certain number of hours during November. The “Combat Tour Offer” awards you Points for the following milestones, as detailed in the FAQ:
Play 35 or more hours, and get 100 Points, OR
Play 70 or more hours, and get 300 Points, OR
Play 140 or more hours, and get 600 Points
The offer also includes a kind of cashback deal, awarding you more Microsoft Points if you spend those you already have on “Halo Game Add-Ons, Avatar Items, Themes, Games on Demand, TV shows or Movies”, as listed on the Xbox Marketplace.
I don't represent Halo 4, so I definitely don't advise that you see this as an easy way to get some extra Microsoft Points. For one thing, to get the 600 for playing 140 or more hours in a month, given that it's already the 7th you would have to play for six hours a day, every day. But if you know you'll be playing a lot of multiplayer or buying some Halo content anyway, and if you're already a member of Xbox Live Rewards, then you may as well keep it in mind. And don't be surprised if you don't win out in the end; the creators of the offer have a maximum of 10,000,000 MS Points to spend, and it's first come, first served.