My 4 year old son constantly complains of sore, tired legs. Mostly when he wakes up or after sitting?

Growing pains. This sounds like growing pains. Massage and warm towels around the legs can help. If the pain is only on one side, becoming significantly worse, or waking your child up at night please see your pediatrician for a full evaluation. Growing pains may last for a few months.
Flexibility. There may be numerous reasons fro chronic soreness. One is extreme flexibility issues. Kids who are not very flexibile thru their joints or oddly enough, extra flexible, often have more soreness and fatigue with exercise.These may be worse during growing periods.Your doctor can help with diagnosis and therapy.
Active or sedentary? The term "growing pains" is often applied to very active kids whose discomfort seems to disappear with a little massage( often hurts an inflamed muscle) & never interfere with a play activity. I would be concerned if the kid was sedentary by nature, pain was more often one sided or painful enough to awaken in the night. A diary of events on a calander may help your dr evaluate this pattern.