A: You're correct that the recommendation is that children be a minimum of 4 years old and weigh a minimum of 40 pounds before they use a booster seat. Many states that have booster laws give this recommendation, but some also recognize that there are some situations, such as a very tall or very heavy 3-year-old, where a booster might be used earlier.
Even when your child meets these minimum recommendations for booster use, it's best if you can leave the child in a harnessed car seat as long as possible. Moving from a harnessed seat to a booster is a step down in terms of safety, because the harnessed seat distributes crash forces over more points on the body and allows for less movement during a crash. Four-year-olds tend to have trouble sitting still in the car and tend to move out of position often, which is dangerous in a booster seat. If the child puts the shoulder belt behind his or her back, leans forward often, or slouches to the side, a booster seat isn't a safe option, because the seatbelt can only work properly if the child is sitting in the correct position.
Find the maximum harness weight on your child's car seat and try to keep the child in that harness as long as possible. It's also important to keep an eye on the child's height, as many toddlers outgrow harnessed car seats by height. The harness straps should be at or above the child's shoulders when forward-facing, and the tops of the child's ears should be below the top of the car seat shell. Once the child reaches the height or weight limit, you'll need to look for other options. A booster seat is one option, or you could look at one of the higher harness weight car seats, such as the Britax Regent (harnessed to 80 lbs.) or the Cosco Apex (harnessed to 65 lbs. then becomes a booster).