K.O.
I would have an absolute fit if anyone tried putting my 4 year old in a backless booster. Does a shoulder belt even fit a 4 year old properly in a NBB? Although not ideal, I would understand if a high back booster was needed. A high back booster shouldn't take up any more space than a no back booster. If you are ever in an accident - those are going to be some very non protected 4 year olds.