Hi I was directed her by a friend. I am a child passenger safety instructor. With your child only being 4 1/2yrs old & 44lbs it is best to keep him in the high back at this time. A high back booster does offer some side impact protection and also allows the child to be comfortable esp if he still naps in the vehicle.
Another poster said lowback booster are not safe which I want to clear up. Lowback booster are safe to us so that info that they received is not accurate by any means. If they weren't safe then they wouldn't be sold in the US. All seats (including boosters) must meet federal motor safety standards (FMVSS) guidelines. Each milestone in car seats/booster is a seat down in safety. That is true but low backs are safe but not ideal for a 4 1/2yr old. I would wait until he has outgrown the high back when the booster seat back is fully raised to the top positions & his shoulders go over the red belt guide (where the shoulder part of the seatbelt feeds through on the booster).
If you have any more questions please feel free to ask.
Mom to 2 girls ages 6 1/2yrs old & 4 1/2yrs old forever harnessed in Britax Regents