A 4-year-old child should be able to feed himself. Children this age strive for independence, and if they are provided with age-appropriate tools, they will try to be self-reliant. To foster your son's independence, provide him with small utensils that he can easily handle with his small hands. Expect that he feeds himself, and do not give into his requests. If he does not eat a whole meal because you did not feed him, let him leave the table. He will be hungry when the next meal is served!
A parent's job is to provide a nutritious meal. A child's job is to eat it. Schedule an eating plan: breakfast, lunch, dinner, snacks in between, and a bed time snack. That way he will eat every 4 hours or so, and you will know that he is not hungry. Children your child's age have an inner ability to regulate how much food they need for growth. So don't worry, your son will not starve himself! Encourage him to be independent with his eating habits though.