Under Article 51 of the Road Traffic Safety Law Implementation Regulations, what rule applies to a motorcycle entering a multi-lane roundabout (环形路口)?

1. Vehicles preparing to enter the roundabout must yield right-of-way to vehicles already circulating inside the roundabout (进环形路口让已在路口内环行的车辆先行)
2. Vehicles entering the roundabout have automatic priority over circulating traffic
3. Entering vehicles must sound their horn continuously and accelerate without looking
4. Vehicles already inside the roundabout must stop completely to let entering traffic merge