Doors are objects in SAS: Zombie Assault 4. They come in three different kinds.
Destructible doors lead into black space, and can be opened by shooting at them, revealing loot, except for the instance of a door inside the lab's entrance to the restroom in the mission Vaccine where it will tell you where the vaccine is instead. They can be walked straight through by players with movement of +400% or higher, but it will not trigger the loot drop.
Non-destructible doors usually block the players' progress. The players have to take an certain action to open the door, but a player with more than +400% movement can walk through it but will not trigger the next part of the mission.
Passable doors are those that the player can open and it will close behind them. Destructible instances are like those found in Onslaught; non-destructible instances are like those found in Vaccine's subway doors.