A 4 year old child can usually safely and legally ride or sit in a backless booster seat as long as they are tall enough to fit the seat belt properly.
The booster seat helps the child get high up enough for the seat belt to fit around the child properly.
If the 4 year old cannot sit high enough in the booster seat so the seat belt fits them properly then they must be in a regular forward facing car seat with the harness.
But if the 4 year old will fit in the backless booster seat with the seat belt fitting the child properly then they can be in a backless booster seat.
The seat belt must fit the child properly but if you can it's best to keep the 4 year old in the car seat with the harness as long as possible because the car seat with the harness is much safer at protecting your child.