There are four-year olds who have done liver cleanses, but normally I don't recommend them to kids at that age. It is better to approach this problem through diet. Vata-decreasing foods are relieving constipation and should make up most of their diet (see details how to balance Vata problems such as constipation in The Key to Health and Rejuvenation, ener-chi.com) Products like oxy-poweder may be a better choice. Make sure to give him enough fats, including unsalted butter and cold-pressed, unrefined oil, unrefined sea salt and fruits that have some sour taste to them (eaten not with a meal, though, but separately as a snack half way through the morning or afternoon. Eating cooked vegetables at lunch and at dinner is necessary, and so is drinking at least 4 glasses of water per day.