My daughter is 4 1/2 and even though sthe technically meets the requirements, I don't think she is ready for a booster seat. Remember that with a booster there is nothing holding the seat in the car except the child's weight. You have to be careful to buckle the seat in even when you are not using it so it doesn't go flying if you have an accident. Your daughter also has to be mature enough to wear the seat belt properly so that it can effectively restrain her. No slouching in the seat or moving the seat belt out of position. My 6 yr old son is in a Britax Parkway booster and he loves it. Even though it can be used at 30 pounds, Britax recommends not using it until 40 pounds. There is also a mini
mum height requirement but I don't remember what it is.
My daughter is still in a 5 point harness. She is in the Britax Regent which holds a child up to 80 pounds. My son would still be in his Britax Husky (the older version of the Regent) except that the LATCH system in our van has a lower weight limit (I think it is 40-45 pounds). When we tried to install the Husky using the seat belt, it did not feel secure so we moved him to a booster. I use his old Husky for when I am transporting one of my daughter's friends who is under 45 pounds (we re-installed the seat with the LATCH). We love the Regent otherwise. Just check what the weight limit is for the LATCH system in your vehicle (we did not know to do this) and test-fit the seat using the seatbelt instead of the LATCH. If the seat is secure with the seatbelt, you should be able to use the seat until your daughter is 80 pounds and will not need a seat at theat time (assuming she is 9 yrs old or 5 feet tall)
I know that most car seat manufacturers recommend the 5 point harness for as long as possible and I agree. Good luck. Hope this helped! Check out the Britax seats at www.britaxusa.com They are expensive but worth the money!