L.A.
Children know and hear a lot of things we do not think they know.
I knew my mom was pregnant when I was 4. There were just signs. Phone conversations, the way she acted. The way she started talking about other children.. etc.. I think children are many times underestimated.
You are probably very close to your child and she knows you so well, she is intuitive as well as very bright and observant .
She is going ot be thrilled until the baby comes home and interrupts your daughters life.
My mom says the first night my baby sister woke me up, I told my mom she could take "baby sister"back to the hospital". I was over her real quick.
BTW, Congratulations!