Assessing young children is often a challenging process, in part because young children are less schooled in test-taking. Obtaining a good measure of their abilities requires a professional who is skilled in working with young children, who knows developmental levels, and who can establish rapport with young children. Young children often change quickly, showing new abilities today that were absent yesterday.
Before making a decision about placement, I would recommend that you consult a pediatric psychologist or child clinical psychologist who is experienced with young children. They can do a comprehensive assessment of your child, perhaps in combination with other professionals to provide a complete picture of your child's development physically, socially, cognitively, and emotionally. Share with the psychologist your concerns about ADD and your pediatrician's assessment of your child as very intelligent. Sometimes a local public-school district will provide such services for preschool-age children. You may want to contact the school psychologist or principal at your local school for assistance. If the school does not provide such services, you may find a pediatric or clinical child psychologist through a local university or medical center. Your pediatrician's office may also make referrals.