Under Article 68 of the Road Traffic Safety Law, when a passenger bus breaks down on an expressway and cannot move, where must the Warning Triangle (三角警告牌) be placed behind the vehicle?

1. 5 to 10 meters directly behind the rear bumper
2. 50 meters behind on ordinary urban streets
3. On top of the bus roof next to the air conditioner
4. At least 150 meters behind the disabled vehicle in the direction of oncoming traffic, with the hazard warning flashers on