You can start throwing down in Halo 4 multiplayer matches tomorrow, but 343 Industries has already detailed a major update coming to the ranking system early next year. The "Competitive Skill Rank" system will assign you a skill rank, fully visible on your Waypoint profile, and use it to match you in games with opponents of comparable skill ranks.
The announcement notes that you'll get a unique CSR for different game types, so you may be a level 25 in Slayer games and a 30 in CTF games. Right now, the max level is planned at 50, but 343 says that's subject to alteration. And of course, your CSR can raise or drop as you play more matches. All competitive playlists are said to support the CSR system, but playlists will change during the course of the game. The developer also hasn't determined yet if it will use existing data as a baseline for your rank, or if everyone will start fresh when it launches.
343 doesn't detail an exact date for the launch of the CSR, but warns to "set your expectations for release timing appropriately since this will require delicate tuning, testing and implementation." Part of that careful implementation is that the rank will be integrated into Waypoint on the web, console, and Smartglass.
Check out our review of Halo 4 for more details on the game.