There's been a ton of new info regarding Multiplayer. It looks like they are finally going crazy with all the options to really customize your player. Some of the new armor designs are awesome.
Here's the basics from IGN's Halo 4 page
Halo 4's Multiplayer is called Infinity, named after a massive 3 kilometer-long vessel (called the UNSC Infinity) sent into deep space stocked with Spartans, who battle in training sessions. There are two types of multiplayer modes now, War Games (Deathmatch, Capture the Flag etc.) and Spartan Ops (a co-op, episodic mode). Here are some changes made to Halo 4's multiplayer:
Halo 4 will have loadouts called Specializations. Rather than everyone rolling out with the same weapons online, players bring customized gear into battle. Everyone packs a primary and secondary weapon, armor abilities (including Active Camouflage, Jetpack, and Hologram) and a grenade type.
Weapons still appear around the map, but they appear in random spots via drop pods.
The respawn time in Team Slayer is instantaneous rather than reliant on a clock.
Sprinting is an innate ability everyone can use.
Players can also use an ability called Forerunner Vision to peek through walls, Batman style. The X-ray vision upgrade is an armor ability you'll equip.
Abilities are unlockable via Spartan Points, the currency fueling the in-game economy. Points unlock additional Ops missions, armor abilities, and cosmetic upgrades.
You rank up using XP
1-50 ranking system
Multiplayer is somehow connected to the single-player campaign
There is no longer a Firefight mode
Spartan ops is an episodic multiplayer mode that will span several seasons and guarantees hours of play
There is no armour lock
You can now join games in-progress
As for the "Specializations" - these are sort of like different skill based classes, like Battlefield and older CoD. Sounds interesting. This is certainly the biggest overhaul any Halo game has had. I think I'm gonna do a midnight launch. Can't wait.