Epsom Salts are NOT necessary for a successful flush. I have done 5 flushes withOUT them. Just have your child eat VERY lightly....fruits and veggies with no fat or oil; some carbs with no fat or oil. No eating past 4 or so, take the oil/juice before bed (in your child's case I guess that would be at 8 or so).
Have your child take about half a bottle of magnesium citrate the next morning. That is what will get their bowels moving. Be sure to try to get your son to get plenty of plums, prunes, whatever throughout the day to really keep their bowels moving. You can get magnesium citrate in the laxative section of your drug or grocery store.
I'm curious though, why give your 4 year old a flush? Is he having allergy/health problems?
Julia Chang at www.sensiblehealth.com has had families do the flush with their young children. She told me that 30-40 stones were removed from a child of 4, and that the severe allergy to peanuts the child had was totally eliminated.