I apologize for the low quality of this video (and music), but it was the only version I could find.
The player is using the Battle Rifle and you can see the jetpack icon at some points in the video. Some key things to note are the instant respawns (for slayer), assassinations, and they are playing on the map Warhouse.
After comparing this gameplay to the already released videos of Halo 4, it looks to be the real thing.
Also am sure they filmed this with a potato seeing as they are playing in a barn.