All states have the same child restraint rules (apart from taxi rules). And it is perfectly legal to move from an inbuilt harness to a booster seat under the age of 4 if your child is too big for a seat with an inbuilt harness. Using an inbuilt harness when you are too tall is dangerous because of the extra downward force on the shoulders. I know of people that have had to do it, and while not the best feeling it is legal.
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