Halo4’s campaign will not have any flood in this time around, as its place has been taken by the forerunners.
The main reason for them not being in the campaign, is that i think that they were a pain in the arse.
As in many occasions you were fighting off what seemed to be endless waves of the flood
But the flood have not been cut out of Halo4 completely, they will be returning in multiplayer. They will be appearing in a new re-vamped version of infected.