While you wrap your head around that idea, work in this interesting quote from the Overkill game director Ulf Andersson.
"We are working on a very cool blend of Payday and Left 4 Dead. I am sure it is so exciting that it will have some players check into the hospital before we are done."
I'm not sure I even want to try to decipher what that means. I'm too busy sorting out what this crossover would entail. The 4-player co-op is a given, but will it be about bank robbers in clown masks who have to fight off zombies during their heists? I'm not sure if the concept will cause people to check into a hospital, but it is interesting to say the least.