I have a peanut, too! My DD didn't hit 40 lbs. until she was 6. Currently 9, she's barely 50 lbs. and about 4'1" tall. She'll be in a booster seat for a lonnng time yet. LOL!
DS (11) just recently started riding without a booster seat. He's finally 4'9" and passes the 5 Step Test.
http://www.carseat.org/Boosters/630.pdf