S.A.
Personally, I think a four year old is too young to be learning about the uterus and other parts - let her enjoy her childhood.
I told my son (when he was around 4)that babies are made when mommies and daddies love each other so much that they have too much love and the baby is made from that love. (that was a fun topic, considering Im divorced from his father!) I also told him that babies grow in mommy's tummy to protect him/her until s/he is big enough to be born. My sister is pregnant and he has asked how babies come out of the tummy (he was afraid it was through the bellybutton), so I told him that babies come out of the woman's privates (he's six and well aware of the difference between men and women), but that he would learn more about that when he's older. He seemed content with the answers I've given him. Share what you are comfortable sharing, and remember she is only four.... she isn't really asking about "sex"... she's just wondering about babies!