[Update: Here's the Halo 4 trailer.]
Microsoft's keynote just wrapped up with a teaser for Halo 4. In it we see Cortana (who sounds like she's voiced by someone else) wake Master Chief out of cryo-sleep. The ship the two were on at the end of Halo 3 is exploding and Master Chief quickly makes an escape to the outside of the ship.
Master Chief picks up a new gun, which looks like a mini-grenade launcher type weapon and we see that the spaceship is being attacked/pulled in by some giant ship.
Halo 4 is the mark of a new trilogy for the Halo series and it'll be out Holiday 2012.