The best way to teach your kids about nutrition and a healthy lifestyle is to live it, or "practice what you preach". Children automatically learn from the examples set for them by their parents, so if you're setting good examples, you're already ahead of the game. Educate yourself, and your children will benefit.
In 4th grade, kids need to learn about eating healthy foods like vegetables and lean protein, and avoiding unhealthy ones like trans-fats and processed sugars. The new Choose My Plate initiative is helpful to review with them, because it also emphasizes portion sizes and a big part of meal planning, not just eating from the right food groups.
There are lots of books at the library to use at home, or your can find something a bit more formal. Nutrition for Healthy Kids is one choice for a curriculum that teaches nutrition concepts with fun activities throughout. Google can be a great resource too, so don't be afraid to search for what you want your kids to learn about and do some investigation on your own. Remember, if you learn about it, your kids will benefit too!