I did not teach my son to do this until he was six, (this summer) I am so picky about making sure he stays clean while wiping and I wanted him to be old enough to be able to be careful about not getting anything on his hands. Even he will wash his hands, I would rather him not have to open the faucet with really dirty hands and getting the faucet dirty for the next person to use it. I remember this being a major problem for me as a little girl. I don't remember how I was taught, but I always got my hands dirty. Also I felt that is was not really necessary for him to learn until he was in school all day. I spent the whole summer supervising him, showing him how much paper to use, how to fold it, what it should feel like on his bottom when he wipes, so he knows he is wiping the right place and how to avoid "speading" the mess to a part of him that he may not normally wipe. I would always check to make sure the wipe comes out clean, and then he is done. He has learned that sometimes he will be clean faster and other times it takes longer, but he is not done untill the wipe is clean. He uses toilet paper first and finishes off with a wipe. If the wipe is dirty he must keep going until it is clean. I don't feel that a four year old is capable of this kind of thoroughness. I did this with both of my boys. It was hard work for me (what part of parenting isn't)but worth it.
A.