Under Article 81 of the Implementing Regulations, when visibility on an expressway drops below 50 meters (the '520' rule), what must the driver do?

1. Stop immediately on the main carriageway and wait for police escort
2. Turn on all fog and hazard lights, reduce speed to not more than 20 km/h, and exit the expressway at the nearest ramp/interchange as soon as possible
3. Drive at 40 km/h in the leftmost fast lane to avoid slower traffic
4. Follow closely behind the vehicle ahead within 10 meters to use its taillights as a guide