Will you need the car seat in Honolulu? It's very easy to get around on transit, fwiw.
I'd lean to no. How close to 4 is he, and how big? The CARES would work very well for you but I think you are close to topping out of it. Would you consider just the lap belt? We started using just the lap belt once my son turned 4 (at which point he was 40lbs/40", if that is a useful point of comparison).