According to AN ARTICLE FROM IGN, John-117 will have a new look in Halo 4. Half of this is attributed to artistic evolution. Pretty much meaning they just changed certain patterns in the armor to fit with the newer graphics engine. This applies in the change from Halo 2 to Halo 3, where Master Chief is wearing the same armor in both games, but small things change such as the glove or boot designs.

However, other stuff has changed that they aren’t mentioning, more tied to the plotline. Which could have to do with why he had a jetpack that he hasn’t used once in all of the original trilogy and just out of no where decided to use in that trailer. That was a bit of a turn off but then again, trailers are never completely canon, but just meant to give you a  general idea of what to expect, and the game can change significantly between the release of that trailer and the release of the game.

Cortana now… now that’s a touchy subject. I get that she’s nearing the ending of her life span but… you know… I’d really prefer them to save her. They probably will. I mean, the freaking Huragok are organic super computers that can take a car apart and then put it back together within a matter of minutes THE VERY FIRST TIME THEY’VE EVER SEEN THAT TYPE OF VEHICLE.

They could easily conclude the Cortana dying plotline just by having Chief hand Cortana over to a friendly huragok and it fixes her up and makes her immortal… but that’s a bit anti-climactic. They’ll probably make it a bit more epic…assuming they decide her fate is to not die. :X