The best car seat for a four-year-old is not the same as the one that is right for a two-year-old or even a one-year-old. Most seats sold for babies and toddlers have very little support and are extremely difficult to use.
Advent looks different when you are three feet high. Grownups do weird things. And your little brother makes no sense at all.
(December from the perspective of Timmy, a three-year-old boy.)