How should I explain to a four year old why we can't buy her an expensive doll?

My granddaughter is starting to get fixated on a doll that is a little too expensive for us to buy right now. We told her it is expensive and she said "I can buy it, I have money." It turns out she meant she had a little money in a piggy bank but that piggy bank is not going to be enough for her to buy it. I know how women are supposed to see themselves as valued so I want to explain it in a way that doesn't make saving money look like a bad thing. I don't want her to think that saving is a bad thing when saving is a good habit to have.