You should never be able to open four gates. What happens is that when on player wanders too far out of the district where the other players are, the other players warp to the wandering player's location.
... and before you ask
: If you try to block the gates while they are open they will indeed stay open... but only until you force the game engine to de-load that area so that it can load in a new one. When this happens, the engine de-spawns any objects that are blocking the door and places them back at their original spawn points, then closes the door.
But you know what, Enigma? You have made me think of a possible exploit. And while I was thinking about that (and doing some game testing) I decided I have an addendum to Gothi's theory about the loading glitch. So this is going to require another separate Ghost Chasers topic. Oh, man... this coffee really has me going!
Thanks for the inspiration, Enigma!