Under Article 51 of the Implementing Regulations, what is the right-of-way rule when a motor vehicle enters a traffic roundabout (环形路口 / 环岛)?

1. Vehicles entering the roundabout have priority over vehicles already inside
2. Vehicles must stop and wait for a police signal before entering any roundabout
3. The vehicle traveling at higher speed has right-of-way
4. Vehicles preparing to enter the roundabout must yield to vehicles already circulating inside the roundabout (进环岛让岛内车辆先行)