The first two screenshots for Halo 4 have apparently been leaked online. The two screens show only the revamped Master Chief and seem to try to showcase the game's new lighting engine.
Details are still scarce on the game, but with GDC kicking off this week and PAX East kicking off at the beginning of next month, you can be there's going to be a media storm of information on the game soon. 343 Industries took over the Halo franchise from Bungie and are creating a brand new trilogy. Right now Halo 4 is set to be released this fall.
Update: Now in video form!