Aww, I think it's kind of sweet! I'm sure she's just experimenting with the concept. I am pretty sure that all kids that age have those sorts of feelings, but many probably don't label it as such. When my older son was around four one of the moms in our little group of friends hired a nanny that all of the kids had a crush on in one way or another. They all just wanted to be around her as much as possible. It was such a treat for them! There wasn't really anything romantic and certainly not sexual about it, but I swear they all got a twinkle in their eye talking about her.
Since then, my older has never admitted to any "crush" type feelings (and we go out of our way not to push that on him, of course) but there have definitely been a couple of girls over the years that he has talked animatedly about, in a way that is just different than when he's talking about his other friends. One of them was probably at its peak when he was about six, I think. They are still great friends and very sweet together, but I don't think that HE has ever labeled it as any kind of boy/girl thing.
I wouldn't worry about it at all.