Someone touched my son.

My son is 4 years old and since he could talk we have taught him the correct anatomical words for his privates and have taught him that no one should touch his penis besides himself or me and his father when cleaning him. Well today he was using the bathroom at home and he comes to me and says “a boy in a green shirt touched my penis”. I was taken back and ask him again. Then I asked him to show me and he just moved his penis to the side and told me the person helped him move it.

I didn’t keep prying and asking questions because I don’t know how question a 4 year old about these kids of things. I am planning on talking to his teacher about it but how do I go about it? I don’t want to seem like one of those parents who accuses and makes false accusations but my son has never lied or made up stories. I’m so sick to my stomach because this is my first child and I don’t know what to do. He’s not acting any different, I mean he’s been having his hands in his pants a lot more but that’s normal with kids his age.