At not even four he is much, much, much too young for a booster, mama!
I like to see kids at LEAST five years old and at LEAST 40 pounds before parents even think about putting kids in boosters for short trips.
A great seat for you would be the Graco Nautilus. It harnesses to 65 pounds, and then converts to a high back booster and then a backless booster. It does have a recline function