It could be that 343I only gave them the files to CNC the Halo 4 MC. I know Bluerealm has the MA37 file, and probably CNC'd one to go with the suit.
Somewhere on here someone posted the alpha build of Halo 4 (Which was confirmed by 343I as an alpha build) and a MA5C was carried by a ODST.
Looking at the Halo 4 Master chief holding the rifle, there are some noticible differences on the rifle from the Halo 3 MA5C
Differences are that the ejection port returns, there are screw holes on the cheek rest, replaced the straight barrel with a flash hider, looks like it has a ambidexterous charging handle slot, it may have a trigger guard, it may have a collapsable stock and theres a slight angular wavy design on the cowling. We may be looking at the MA5D, which don't look too bad, but I still prefer the MA5B.
343I talked about changing up the weapon gameplay, and I'm speculating that they might make it so the weapons are upgradable, either like Modern Warfare or Crysis?
I'm just hoping that they don't nerf the shotgun.