If the boy can communicate well enough, you can ask him casually if there is anyone who has been touching him inappropriately in his private areas. A 4 year-old usually doesn’t lie about such things, so you can get a pretty straightforward answer from him. Also, if you are the parent or someone who is really close to him, you can probably “sense” that something is wrong, that he is not behaving in his usual manner or in a manner expected of a little 4 year-old. He may cry more often than usual for no reason, or seem depressed, quiet, and sullen. Or he may exhibit sexual behavior that is not normal (especially for a child), like attempting to touch someone else in an untoward way, or making lewd comments or using foul sexual language.
But be very careful before you accuse anyone of molesting the kid——you have to be certain that you have the facts before you try to “handle the manner in the family”. Otherwise, you can create more heartaches and lifelong distrust than ever.