J.S.
I like the toy idea. I like what PB&J did and had him pick out the toy, carry through the store, bring it home, and then put it on the back of the toilet. He'll want that toy that's just staring at him in the bathroom! Although it thoroughly grosses me out to think of a 4 year old cleaning out their own poopy underwear, I think you may need to try that too...with an over abundance of antibacterial hand soap after! But whatever you do, stop enabling him by giving him diapers!