Think about it…
Every Halo game’s co-op but Halo CE was cannon, Halo 3 had you as the Arbiter (this even changed some levels like Cortana) or a random Elite, in ODST you are just a random ODST and in Reach its the same (just a random SIII). I don’t think there will be any random ODSTs/Marines on Master Chief’s half of the Dawn. This brought me to the conclusion that either Halo 4’s co-op will be non cannon, or one of two things happen during the game:
We meat up with either Covi Separatists or UNSC troops at the VERY BEGINNING OF THE GAME. There could be a cut-scene at the beginning that skips lots of time until MC is found, or the planet could already be inhabited by Covies or UNSC troops.
Players number 2-4 will play an unknown alien race or machine (imagine playing as a monitor in campaign 0.o).
Of course it could just be non-cannon. I doubt 343 will sacrifice Co-Op canonocity for believable and planned story, instead of them being pushed to a corner just because of co-op.