Oh gosh, from a Mom's perspective, this is somewhat normal. Pleeeease don't jump the gun and call social services or anything until you've at least spoken to the girl privately and her parents privately. I once had a teacher call social services because my daughter had been out of school for 2 weeks straight because of lice. The teacher was right out of college, didn't have kids of her own, didn't call me to ask why, just called them and I got a knock on my door in the middle of the day with an accusation of abuse. That whole fiasco turned my life upside down. Monthly visits, phone calls to my family and employers asking if they'd ever seen abuse...it was an embarrassing mess, and one that was come tell unnecessary.

So anyway, this girl is still just a baby. 4 years old is too young to really understand what she's doing or why she's doing it. She just knows it feels good. She may have seen something in a movie or she may have just stumbled upon it accidentally. I was maybe 6 when I discovered my genitals and that rubbing them felt good. It's pretty normal, in my experience with not only myself but with my 3 daughters.

If I were you, I'd just pull her aside and let her know that what she did was inappropriate and that those are private parts. Tell her it's ok to touch them, but not at school. Or something to that effect. You know what she'd respond to better than I do.

And just to be sure there isn't anything strange going on at home, give the parents a call and see what they have to say about it. Keep an eye out for other signs of neglect and if you're noticing multiple things, maybe schedule a meeting with the parents and the principal and if warranted, call social services.