M.S.
Just to correct the poster - the law is over 40lbs and 4 or over (a child who is 3 and 10 mo is considered 3) for a non-harnessed booster seat. Most states require three levels of car seats: 1. Rear facing car seat for all children under 1 or under 20 lbs
2. Forward facing harnessed car seat for all children over 1 and over 20 lbs but under 4 or under 40 lbs
3. Booster seat if over 4/40 and under age 8 or under 60 lbs.
The last recommendation varies from state to state but is recommended by all of the child safety groups/AAP/etc. The key is, don't fudge. I have two kids of my own, and it is a royal pain. Especially when we are just driving from one store to another, and I have to buckle them both in to go across the street! But I do it, every time, because what if I didn't?
Keep them in a five point harness until you can't. My 3 year old is pretty big, we moved him from his Britax Marathon to a convertible seat (booster with a 5 pt harness) made by Graco. We needed the Britax for his little brother, who outgrew his infant seat. The Graco is not nearly as good as the Britax, he barely fits in it (he's 35 lbs), but he will stay in it until we can afford a better 5 pt harness seat that is rated up to 65 lbs. At 4 I think he will still be too little to be in a seat belt.