C.,
The sooner you tell her the better. The longer you wait, the harder it will be. My husband was adopted by his stepfather. Children have an amazing ability to handle situations that adults think they never could understand or cope with. She is still so young that if you are matter of fact about it, she shouldn't have trouble accepting the fact that her daddy came into her life as a baby, not a newborn.
There are lots of "I'm adopted" type books and videos at the library and the bookstore you may want to try. Talking to a school counselor or therapist for ideas and wording may help you, too.
We have a lot of friends who have adopted children. Many of them like to repeatedly tell their children the bedtime story of how the child came into the family. They include the details that make the child feel special. Examples: I was so excited to be your daddy. I feel so lucky that Mommy said that I can be your daddy and help raise you. It is such an important job. That is why I adopted you, so everyone will know you are mine. The day I went to sign the papers was so special. The kids love this.
If the biological father chose not to be in your daughter's life, don't demonize him. Children take this personally even if they have never met the father. Just make it matter of fact. (It is too bad that he decided not to be a dad. He is sure missing out on all of the happiness Mommy and Daddy get from raising you.)
This is a tough one. Best of luck to you,
S.