Thanks to user Xepler who took this image of the supposed Halo 4 Fuel Rod Cannon. He has an early build of the game, which this was taken from. At this point it looks very similar to Reach’s.
There are some minor modifications to the design of the weapon besides the neon blue lights and the dark finish. He says it is the placeholder for the fuel rod from an earlier build of the game. Remember this is not confirmed.