Don't give him options. If he has options, then he has an opportunity to get his own way - which is to wear a nappy. At 4, he is more than able to understand what you are trying to get him to do. What he did just now, he doesn't realise that you are deadly serious about him doing it. You are going to need all the strength you have to not cave in on this, so you will need to be prepared for lots of fights and tears (and that's just from you! ) and most importantly, it might take a week or more, but you have to stay strong throughout it. If DH can't be as strong as you, do it on your own and get him to help you with other things instead while you concentrate on doing it.