No mention of 4-player co-op, and the way the story is structured, it doesn't look like it will happen. I guess the 5-player co-op Horde mode is an apology of sorts for no 4-player co-op.
And judging by tidbits, it looks like Dom is going to die of Iron Lung in this game. He was coughing at lot during the E3 co-op playthrough.

