He's 4 years old and a boy...not that unusual...boys don't always train as easy as girls.
When my son was 3 years old...he messed his undies...January..26 degrees out...I took him out to the front yard (8 am)...ran the water hose down his undies...he's yelling at the top of his lungs...out comes the neighbor (she had 2 teen sons at that time)...and she's laughing like crazy...why?
My son is yelling ...MORE! MORE! MORE!.... Mary says..."I could have told you that wouldn't work...they like it too much...and you just made a bigger mess to clean up."
I have pics of my son, in a t-shirt, with the hose, watering a tree...blue lips and all! Yep...took half an hour to get him to come in and take a bath.
They need to just give him a few rules... no pooping at school...only at home...and then he will clean himself. Trust me ...that will work better than anything...it's messy...but just messy enough that they will decide not to do it any more...and not doing it at school...soon he will realize that using the potty is not a bad thing.