Not sure mate, definitely before the 31st though.
Co-op horde is my way to game mate, no question. I even played GTAV like a horde mode, get in the car with a couple of mates and simply raise hell with the Old Bill/Home Boys/Military. The single player didn't interest me at all.
Looking at Swanny's AC 4 video was tempting though, just like FarCry 3 did the single player with crafting etc. Still wobbling over whether I'll like the controls though. I could probably pick that up for 25 quid.