What is the requirement of mandatory sign 4.6 'Minimum Speed Limit' (blue circular sign with a white number, e.g. '50')?

1. Driving at a speed higher than 50 km/h is prohibited.
2. Driving at a speed lower than 50 km/h is prohibited (except when stopping or turning).
3. Recommended cruising speed is 50 km/h, but any speed is permitted.
4. The sign applies only to heavy vehicles during night hours.