PP is mostly correct...however, SOME dedicated boosters seats come with the ability to LATCH the booster to the car in the same manner as a harnessed car seat is installed. (Some examples are the Sunshine Kids Monterey and the Clek Oobr, mamalori is not correct the the Parkway is the only one) And SOME combination seats (such as the Britax Frontier and the Graco Nautilus) allow you to continue to install with LATCH when using the seatbelt with the seat in booster mode. This is simply to keep the booster from becoming a projectile in a crash.
For anyone reading, if you have a booster seat or a seat being used in booster mode you should rebuckle the seat belt over the seat when the child exits the seat for your safety and the safety of any other passenger in your vehicle.
OP, at 4.5 your son is likely not quite mature enough to handle full time boostering. If he has outgrown his convertible seat, and your budget can handle it, I would get a combination seat like a Graco Nautilus and harness him a while longer. If you cannot do that, then I'd do as the pp suggested and lock the seat belt for a while.
Coleen
CPST