at 4, a child doesn't need to LOSE weight. they really just need to not gain any MORE weight while starting a healthy lifestyle. they will continue to grow UP and lose their percentage of fat and build up muscle. he might lose some weight in the process, but the focus should not be on that, but on eating healthy and exercising and making good decisions when it comes to food. there is nothing wrong with an occasional treat, but most foods should be whole grains, veges, fruit and protein. if there are only healthy foods in the house, you can let your son choose one thing he wants to eat and you KNOW it will be healthy. for lunch and dinner (unless i cook a meal) i let my daughter pick one thing she wants (cheese, crackers, banana, etc.) and then i add 2 things from 2 other food groups and she usually ends up eating all or most of it. i also make it a point to eat healthy snacks WITH her, such as apple slices with cheese or carrot sticks with ranch. i also let her "help" me make meals and she loves to try each ingredient as we add them. then when the meal is ready she can see "oh that's the broccoli i tried earlier". sometimes i also "bribe" her, like i'll give her 2 or 3 chips with half a sandwich and say she can have a few more chips if she eats her sandwich.
i would recommend talking to your pediatrician to find out how many calories your son should be eating and how much exercise he should be getting. i believe the serving size for toddlers is one tablespoon per year of age. so 4 TBSP for a 4 yr old
hope some of my ideas help you out