Ubisoft just showed off gameplay for Assassin's Creed Unity for the first time, and as part of the reveal they announced that there will be four-player co-op.
This is the "biggest playground ever. Hundreds of quests,
The Assassin's were shown off walking through an extremely large crowd of characters, and the visuals were quite impressive. As they should be as Unity is next-gen only. We'll have the gameplay video and hands-on impression later today.