My four year old son has lost a lot of weight and is not eating well. He is otherwise very active and playful. Should I take him to the doctor?

If he is active and playful as you say, there's is nothing to worry at all. Some kids grow leaner and taller, perhaps your child is one of them. As a mother of a four year old I have understood that kids have their own phases in which sometimes they do not eat well, feel way more hungrier than usual and are cranky most of the times. However, discuss it with your doctor the next time you visit him.