4 IMO because after this, Chakra takes the shape of an aura around the user and lot of stress befell upon the user... And Kakashi can't handle such stress and chakra drainage...
P.S. Technically he can open all gates, all he needs is skill which he has...
Because opening a gate increases the flow of chakra from a specific pressure point, it all depends down on how much time will it take to drain out all chakra...
In case of Kakashi, once he opens all gates, he'd be empty right after the opening of the 8th gate... and Dies.