Well, it seems the cat may be out of the bag now due to a bit of a blunder. If the information in this IGN article
is any indication, Microsoft will be announcing Halo 4, as well as a remastered edition of Halo: Combat Evolved, at E3 today. Their conference starts at 9am Los Angeles time (that's 5pm UK time).
Keep a close eye on those Twitter feeds over the next hour or two.