Simply because it is not a prescrition drug, and is manufactured from a natural substance, is not a reason to give it to a child as young as four in any case.
Diagnoses of ADHD
as four have to be suspect - after all, the core symptoms
(overactivity, inattention, impulsivity) normally occur to some extent in most four year olds. If your child, though, is extremely hyperactive (i.e., at the severe end of the spectrum), Dexedrine might be considered.