You may be starting your Halo 4 multiplayer career tomorrow, but 343 has things in store for fans in early 2013. A major update will be adding a Competitive Skill Rank (CSR) system to your profiles, which will assign you a skill rank number, visible on your Waypoint profile and will be used for trueskill matchmaking.
Mirroring Halo 3's changed skill ranks, each individual mode will be measured differently, with varying skill numbers based on your performance. It's not a locked in number by any means, and you can progress up or down as you play more matches. 343 has not decided if your overall performance will be considered, or if you will start fresh once it launches. The skill ranks are set at 1-50 which is open to change between now and then.
Don't expect an exact release date anytime soon. 343 warns that it is a delicate bit of coding, especially as it will tie-in the Waypoint, Xbox Smartglass, and your console.