Our son is going to be 6 in a few months, is about 4 ft. tall and 45-50 lbs.
He sits in a Graco Nautilus -
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It looks kind of monstrous in the picture, and it is larger than some, but it's good for kids up to 100 lbs and has a lot of safety features, including a five point harness. He finds it comfortable and likes the cupholder and pockets.
It fits easiest and snuggest in the rear passenger side seat. I lift the Insight's rear headrest and run the back strap under it to the anchor point in back. I use the LATCH system for the base.
It lists for $180 but we got it for a fair amount less. The children's hospital in our city has a "safety store" that sells safety items like car seats, helmets, knee pads, etc. for not much over cost. I don't know how common this type of shop is but I think it is worth looking into to see if your area has something similar.