of the Spartan Ops mode gives a little bit of detail on why there are Covenant enemies in Halo 4
As the three mile-long Infinity drops out of slipspace it rams through the Covenant capital ships and opens up its MAC guns on everything it can't park on. This particular Covenant fleet is manned by the surviving religious zealots - the Grunts and Elites who remained loyal to the Prophets even in the face of extinction.
Frank O'Connor is comparing the Spartan Ops DLC missions to television shows, suggesting that if the mode is well-received additional "seasons"
could be released after the missions currently planned.
OXM UK has a list
of five ways in which Halo 4
multiplayer is different from past Halo