Our daughter is getting to big for her car seat as well in my husbands spare car. The car seat was given to us from a good friend who is a police officer. This officer is a trained car seat insatller/inspector. My husbnad asked him just the other day about putting our daughter in a booster seat for his car only. (Our other cars have Britax Marathon seats so they are okay for now) The officer said it is 4 years of age or 40lbs before a booster seat. Well my daughter will be 4 in Feb and she is only about 32 lbs now so most likely 4 will come before the 40lbs. I am not worried about her safety if she is not 40lbs being in a booster at age 4. Officer said she would be fine as long as the seat is installed properly and it is used properly. Hope this helps!