I have 4 kids, 2 really good eaters and 2 picky ones. I would avoid trying to "hide" good foods in meals. My kids all will at least try something if Mom deems it "healthy". Just like an adult sometimes they find that they like it and sometimes the flavor or texture of the food is not to their liking. As long as they try it, I am happy. At the age of 4 you have a lot of time to wait out his picky stage, it will pass. Sometimes, getting him to help prepare the meal or help pick out healthy foods while you are shopping and help in raising the amount of interest or curiosity kids have in foods. Also, small portions of a lot of variety at a meal will sometime encourage a picky eater to expand their food comfort zone (put out small bowls of raisins, nuts, cubed cheese, sliced lunch meat, yogurt, rice cakes or crackers, carrot sticks, celery with peanut butter, etc...) There is a company, that puts out a free newsletter that is all about healthy eating for the whole family, called Fresh Baby. You might want to check that out or research on the internet for other places that will deliver free info to you. Finally, as long as he is healthy, your doctor has no concerns and he is not under-weight or over-weight, don't panic! Best of luck to you!!!
I have 2 tricks that might help. One is rainbow sprinkles, everything tastes better with sprinkles including oatmeal and broccoli. What worked with my youngest is dip. Didn't matter what the dip was (ranch dressing, onion dip, any salad dressing, ketchup, mayo--whatever) or what he was eating, if he could dip it, he would eat it.