I am a Child Passenger Safety Technician in the state of Massachusetts.
Right now, the LAW in Massachusetts is that a child must be in some form of child restraint to the age of 8 years or 4 feet 9 inches.
The REALITY is that very few 8 year olds are big enough to sit without a booster. Most kids are closer to 10 or 11 before they are big enough to safely sit in a seatbelt alone.
Four years AND forty pounds (absolutely no younger and no lighter) is the BARE MINIMUM for switching from a harnessed car seat to a belt positioning booster. And we don't like to see four year olds in boosters.
Most four year olds are NOT mature enough to sit properly in a belt positioning booster. It's much better to get a seat that harnesses to a higher weight, like the Cosco Apex, Graco Nautilus, Sunshine Kids' Radian, or the Britax Regent or Frontier.
Moving from a harnessed car seat to a booster should not be seen as a 'graduation' or something you want to do. Boosters are substantially less safe than harnessed car seats.
Again, I would NEVER put my own four year old in a booster and when I do seat checks I do not encourage or suggest it to parents. If a parent arrives with a four year old in a booster, I make sure they know it is not even close to the safest option and they need to buy a larger harnessed car seat.