According to Article 81 of the Road Traffic Safety Law Implementation Regulations, when driving a passenger bus on an expressway in dense fog with visibility of less than 50 meters, what mandatory actions must be taken?

1. Turn on fog lights, low beams, clearance lights, and hazard warning flashers; limit speed to no more than 20 km/h; and exit the expressway at the nearest interchange ramp as soon as possible
2. Turn on high beams and travel at 60 km/h in the leftmost overtaking lane to clear the fog bank quickly
3. Stop the bus immediately in the central travel lane and wait for the fog to dissipate
4. Honk continuously and follow the rear bumper of the lead vehicle at a distance of 5 meters