This is according to the Florida Dept. of Transport:
What is Florida's Child Restraint Law?
All children 5 years of age or younger must be properly restrained no matter where they are sitting in the vehicle. Children through age 3 must be secured in a separate carrier or a vehicle manufacturer's integrated child safety seat. For children aged 4 through 5 years, a separate carrier, an integrated child safety seat, or a safety belt may be used. (FL Statute 316.613).
So, only children three and under are required to have a car seat. At least, that's how I understand it.
We will buy a booster for DD5 once we've arrived.