Potty training problem, son is almost 4 - please help!?

what does he do if you leave him naked, or at least bottomless? mine is 2 and it worked wonderfully with him. having a potty chair in the same room as you can help too, then once he's doing it in there move it closer to the bathroom, then in the bathroom, then have a potty seat on the toilet. we also did a sticker chart with two different sized stickers, he got to put a little one on the chart if he peed, and a BIG one on if he pooped, pretty soon he was trying to pee or poop all the time just to put a sticker on the chart.