J.K.
Please, oh please don't put your son in a booster yet. 4 is too young to be in a booster. There are several carseats that harness to 65-80lbs. Most kids don't sit correctly in a booster until at least 5yrs or older. There are a few youtube videos that show you the importance of a 5pt harness like BELLA'S GIFT, KYLE MILLER FOUNDATION, and there's one more I just can't think of it right now. There's graco naulitus, it harnesses to 65lbs, the safety first apex that harnesses to 65lbs as well, but you need adjustable headrests in your vehicle. The graco is around 150 and the safety first one is around 100. There are also others like britax marathon harnesses to 65, britax boulevard also harnesses to 65, britax radian to 80lbs, compass true fit that harnesses to 65lbs, sunshine kids radian has one that harnesses to 65 and another that harnesses to 80lbs. The britax and sunshine kids carseats are a little more though and the true fit is around 200.