Go to the dead center of level 4 (the Atrium) and in the safe room incapacitate a survivor (note you can not do this on easy as there is no friendly fire damage).

Close the door so the incapacitated survivor will not die. Then go to the elevator and start it.

If you do this fast enough the horde will not come when you reach the bottom. Collect all the gas cans, throw them by the car and start the horde by going by the elevator again.

Extra note: Don't fill the car when there is no horde or you will lose!