When is an overrun-braked agricultural trailer permitted to travel at a maximum speed of 40 km/h instead of 25 km/h?

1. When the trailer is type-approved and certified for 40 km/h, the brakes act on all wheels, and it displays a 40 km/h speed plate
2. Whenever the towing tractor itself is rated at 40 km/h
3. Whenever the trailer is loaded to less than half of its authorised mass
4. Whenever the overrun brake acts on the rear axle only