One thing you can try at 4 he is old enough to come shopping and choose some things he would like to eat,I also feel the more they see that it causes Mom a lot of concern and he gets attention the more they play into it.So dont make a big thing about him eating,put his food down and let him get on with it. sometimes I have noticed the more that is made of an issue the more it becomes. I know its worrying, if you scroll down you will see many posts about eating. I had the same thing with two of mine, one was skinny as a rail, I nagged away at her to eat, she did okay and after the phase was over, she ate everything.Good Luck