P.M.
I am so with you, my son is going to be 4 on Monday and he just won't go potty. He totally gets that he's suppose to and has the control he just won't go. He will usually pee when we have him sit on the potty but he hides whenever he poops. If you catch him he tells you to go away or look at something so you don't see him. we have tried taking toys away adn TV away and we also have rewarded him when he does poop on the potty. He just won't stick to it.
I try not to stress about it because i know it makes it worse but i'm so tired of changing him. I tried the underwear thing too and tht just go me more aggrevated because then i needed to clean him, the clothes, and whatever else that he got soiled.
I'm sorry that I'm not offering advice but I wanted you to know that you aren't alone. My friend tells me that noone walked down the weddign aisle still wearing diapers..LOL