Well, this is embarrassing. I just caught my 4 year old fiddling herself down there. It doesn't look like she's deliberately trying to find any sexual pleasure, more like she's just touching her vag like it's normal. Do kids her age do this? Should I worry?

That's very norm! She might even be interested in boy's private parts too. At this age they are very curious jus educate her about the parts, why can't we touch and why can't we let others touch and see too etc. Tell her it's a body part for us to pee, not a toy to play with. And since its our body part, it belongs to us ourselves only n hence its our "property" can't let other touch or we can't touch others too. For the first time she might not get it have to educate her on different occasions.