A 4 year old could use a backless booster seat as long as you have high seat backs and head rests on the seat.
The 4 year old must also fit properly in the backless booster seat and the seat belt must also fit the child properly as well because if the 4 year old doesn't fit properly with the seat belt in the booster seat then they need to stay in a car seat with the harness.
A 4 year old really should stay in their regular harnessed car seat until they outgrow it before being moved to a booster seat.
The regular car seat is much safer and secures the 4 year old much better and usually by 5 years old the child will be ready to move to a booster seat.
I would recommend using a booster seat with a back and harness to keep the child secured in the seat much better.
Also check your local laws regarding car seats because some states may require a 4 year old to stay in a regular car seat and not a booster seat while other states may allow a 4 year old to ride in a booster seat.