Every kid is different. Some kids may be four years old but are either smaller & weigh less than average or are bigger & weigh more. So, it is practical to have a product that can adjust to the growing body of your kid.  A booster seat can be used with all its parts front-facing if your kid's build is small.  And as your child grows, you can get rid of the back & sides and secure them with your vehicle's seat belt while they sit on the base of your product. However, when it comes to overall safety, a booster seat is less safe than a harness.