Both Cullen and Ridley have Safety 1st car seats, I love them! Cullen has the Safety 1st Complete Air 65. It goes up to 65lbs and has an adjustable headrest and positioning as the child gets bigger. The seat can sit straight up, like a high back booster, since Cullen is only 32lbs and I don't see him getting to 40lbs by the time he is 4 he is going to be in the seat for a quite a while longer.
It is a taller seat, but with it positioned straight Cullen can easily climb in on his own (but it might depend on the car you have.) And it might also depend on if he is still RF. We just switched Cullen around, but he could climb in either way.
(its an older pic)