Alright, so I've been digging in Halopedian, and I recommend that people look at the sources on the CE HD and 4 pages. The Mk. V helmet that we saw in the footage was just a pre-final helmet that WILL NOT be in the final game, as put by the almighty Frank O'Connor.
As the handgun goes, we know that the M6 deries of pistols is highly modular, as we've seen 2 variants of both the M6C and the M6G, so it makes perfect sense that the Chief would grab an M6G, add an attachment, and blow some debris to make a getaway. The final scene, with the Chief attaching something to the mystery sidearm, could be hinting at an upgradeable weapons system.
The 'jetpack' looks like it was just an attachment that was always there for use in zero-gravity environments. I believe Mk. IV featured some kind of booster system in The Fall of Reach, but I could be mistaken.
Just my two cents.
EDIT: Also, IGN Rewind states that the trailer for Halo 4 was NOT made using the game's resources, although I have a hard time trusting these guys- their 'facts' tend to be far out.