My daughter is 5 and outgrew a carseat that I loved.  She is tall, but very thin and I did not feel comfortable putting her in a booster.  This is one of the very few carseats that has a 5 point harness that will last beyond the height of a normal 4 year old.

This carseat is pricey, but in comparison to how often it is used and the safety it provides it is well worth the money.  Also it is very roomy and she will be able to sit in it comfortably for a few years.

I have a minivan so space is not an issue, however I do not think it would fit in a car very well.