In generations past, this used to be called "playing doctor" and was dismissed with a parent saying stop doing that.
This is normal curiosity.
But I think, in this time, you need to treat this with more gravity, and meet his head on as this is no longer considered normal, although it is.
If she behaves this curious way at a friend's house, it's likely she'll be banished from normal social circles. This used to be handled with a "kids cut this out right now" but now looking at each other's genitals is considered deviant.
Your daughter is fine. But you need to talk to her, in very very clear terms about this situation, and tell her not to do it again.
Sad.
At 4 years of age children may not know right from wrong. Children do learn from experimenting. Let her know this kind of play is not appropriate. Pay attention to what she sees on tv or on the internet. You should sit and talk to see if anybody has ever touched her on her private parts or in a bad way. When she has friends over keep an eye on them and if things get to quiet something maybe going on. So you did the right thing for now.
At 4 years of age children may not know right from wrong. Children do learn from experimenting. Let her know this kind of play is not appropriate. Pay attention to what she sees on tv or on the internet. You should sit and talk to see if anybody has ever touched her on her private parts or in a bad way. When she has friends over keep an eye on them and if things get to quiet something maybe going on.