four year old cant digest food

I think this problem is beyond anything you can handle on your own. You ned to seek medical treatment ASAP, especially since he is losing weight. I don't mean to scare you but this is a major problem which needs to be addressed immediately. If he is not digesting his food, he is not getting enough calories and, more importantly, he is not getting any vitamins, minerals or nutrient that are essential for body function. Long term deficiency can cause all kinds of problems including organ damage. So, please get him to a doctor.