hipod Barcelona

My ds turns 4 in 2 months time and weighs appr 19kilos.. I have bought the Barcelona car seat from target, it was on sale.. I also bought the H Harness. but when putting it in the car, the seat belt is to small to work with the h harness, so I have just him in the bosster seat with adult seat belt.. I have called hipod and also checked aus laws and from the age of 4 yrs, they can use an adult seat belt if over the weight, but what is safer? what do others use