There is nothing even remotely strange about the fact that a four- year- old child is masturbating. That is completely, 100% normal. Extremely common, even. Some children begin touching their genitals as young as three months old. It’s not a sign that she’s been molested, or that she’s seen other people masturbate, or anything else scary or inappropriate. It’s a sign that she’s a healthy child exploring her body. (My earliest memory of masturbating was when I was about 6 or 7; and in that memory, the experience was already familiar, something I’d been doing for a while.)
And IMO, there’s nothing strange about the fact that she gets mad when you try to stop her. I mean, if someone in a position of great power and authority over me tried to stop me from masturbating, I’d get freaked out angry, too.
It seems to me that your problem isn’t that your child is masturbating. It’s that she’s masturbating in front of you, and possibly in front of other people, in ways that makes those people (including you) uncomfortable.
So let’s deal with that problem.
To find out how I propose dealing with that problem, read the rest of the piece. (And if you decide to post comments here, please consider cross-posting them to the Blowfish Blog — they like comments there, too.) Enjoy!