I wonder...H4 is supposed to take place directly after H3 (or some time after)
Spoiler:
But is it really the Mk VI he's wearing here? I don't remember the stomach rings and chest area looking quite like that...
But then again, it might be just a minor redesign to accomodate the advances in 3D imagery (just like how Bungie added greater detail to the weaponry in Reach because the game engine could manage more polygons than ever)...
But if this isn't the Mk VI, it could suggest a reunion with the UNSC and possible upgrades...