Ok, so I came across this conversation in the Halo book Primordium:
ONI COMMANDER: “I have to say, I’m having difficulty with all this information. ‘Arks?’ There’s more than one?”
SCIENCE TEAM LEADER: “The Halo as described is also larger than any we’ve encountered. That could imply a larger Ark, right?”
ONI COMMANDER: “Hmmh. There’s still a high probability that this machine is a decoy, and all of the information it’s giving us is a ruse. However ancient, the Forerunners might have anticipated an eventual human resurgence, and possible rematch, and prepared for it. To the extent that this testimony could demoralize our troops, we may be playing right into their hands."
This leads us to believe a few things. First, there is another ark. Second, this other ark is LARGER than the first one, considerably so. From this, we can hypothesis a bit. From the available data, I believe that the forerunner object/ship in the Halo 4 trailer may actually be this larger arc. Thoughts?