Halo 4 is mixing some old and new, bringing back the familiar Covenant enemies alongside the more alien Prometheans. The zombie-like Flood have been conspicuously absent, but 343 Industries has confirmed they will be making an appearance in their own dedicated multiplayer mode.
CVG reports that the "Flood" multiplayer variant will work a lot like the Infection mode from previous Halo games. It supports 10 players, two of whom are infected and trying to spread it to the other players.
This seems like a reasonable compromise, between the millions of Halo fans who are probably sick of the Flood, and the two that would be disappointed if they were removed from the game completely.