Bus operators will often find it economical to convert retired buses to use as permanent training buses for driver training, rather than taking a regular service bus out of use. Some large operators also converted retired buses into tow bus vehicles, to act as tow trucks.
Explain to an 8 year old child that is not yet 49 tall that the safety equipment built into the vehicle is for adults  not children, and a child needs to continue to use the proper equipment (booster seat) so that the adult equipment will fit, and be able to protect, their bodies in the event of a crash.
If your state only requires boosters to age 4, your 4 yr old won't be safe in just a seat belt--keep using that booster! Most kids won't reach 4' 9 until they are between 8- 12 yrs old. And kids should stay in the backseat until they are at least 13.