Final answer:
Children under 4 years old must always be fastened in a safety seat or child restraints whenever they are in a vehicle, as per child safety regulations.
Step-by-step explanation:
The question asks about the conditions under which a child under 4 years old must be fastened in a safety seat or child restraints. The correct answer to when a child under 4 years old must be fastened in a safety seat or child restraints is: Always. This is because child safety regulations and best practices dictate that children in this age range should always be secured in an appropriate car seat or restraint system anytime they are in a vehicle, regardless of the journey's length, where they are seated, or whether they are awake or asleep.