All of the Halo fan boys/girls have been actually waiting for some game play footage of Halo 4 and got some earlier today when some footage of the game was leaked. Geoff Keighley got in Reddit and answered questions about Spike TV's coverage of the presser and also revealed aforementioned bit of information. You can check out the AMAA about E3 here
If you want the gist then here's a quote for ya'll:
Halo 4 is a big focus for Xbox. We've announced that you'll see the first real campaign gameplay in the Xbox press conference on June 4. And then that night, in our primetime special we will be showing off Spartan Ops mode for the first time. I was at 343 last week. We'll be sharing more about Halo at E3 soon.
Microsoft's E3 presser will start on June 4th at 1:00 P.M. EST and 10:00 A.M. PST.Source: Digital Gigolo