At four years old, your son is still learning about his body and is likely interested in his brother's body because it's just like his. I think you should react calmly and not make a big deal about a little touching. It's not unlike your four year old touching any other part of his brother; it's merely curiosity. In fact, making the point to tell him that he shouldn't touch his brother there may make it all the more interesting for him. If you feel the need to get him to stop, especially if he touches his brother for more than a little pat or it bothers your two year old, redirect your four year old to something else he would like to do, and get your younger son dressed. :-) It's likely this behavior will fade without any lasting concerns.
