My daughter is four years old, she keeps calling her vagina her "butt." Should I tell her about a vagina or is she too young? I'm afraid we will be out in public somewhere and she will blurt it out!
During the preschool years, kids are loaded with many questions and curiosities. Questions about and interest in body parts, the differences between the sexes and where babies come from are common.
If the interest is there, a straight-forward honest answer is the best response, but depending on the question and the child, explanations need to be simplified and tailored to the developmental level of the child. Kids at four aren't ready for the anatomic details of childbirth, for example, but can grasp the idea of where babies come from.
As to your specific question, calling the vaginal area by its proper name, or even a related nickname, is better than using the wrong body part name. At some point she'll hear the word 'butt' in conversation, and will get confused if she thinks of the wrong body part. If she does blurt out what may be considered a private word in public, well, that's no reason to be embarassed. It just shows that her parents have addressed her questions honestly.