Please answer - age, height and weight questions for car seats?

My four year old (average to tall and skinny for his age) was 36lbs at his four year appointment, I asked the doctor about moving him to a booster around christmas (three months before his fifth birthday) because the law in California, where I live, is 4 and 40 to move to a booster. She reminded me that pediatricians are recommending kids stay in a 5 point harness as long as possible. We switched him into his sister's Evenflo Triumph a href="http://www.target.com/Evenflo-Triumph-LX-Convertible-Carseat/dp/B002C3S0QE/ref=sc_qi_detaillink" rel="nofollow"http://www.target.com/Evenflo-Triumph-LX…/a which is rated to 50lbs. (be careful, some Evenflo models that are the same name and only appear to be different patterns have different weight capacities - his Evenflo Triumph was only rated to 40lbs, so we use it for his two year old sister). I love that the lap belt goes over his thigh bones and not over his hip bones, it's easy to install/uninstall in the grandma's cars, and adjusts easily. We are going to keep him in it until he's a big-boy kindergartner (5 1/2) and then move him to a booster. There are boosters with 5 point harnesses too. Frankly, I figure we've had him in the seat since he outgrew his infant carrier and if it lasts for four years, we've more than gotten our money's worth - even with removable covers that can go in the washer, the mess of muddy clothes from playing hard, accidents while potty training, and snack spillage means that I'll be ready for something new. The booster law where I am is 6 and 60 so we'll probably keep him in his booster at least until he's at the end of second grade and he'll want a cool one that isn't the same one he's had since he was a baby..