Telling your daughter the truth

Hey guys. Ok. My daughter real father is not in the picture. Hasn't been since she was 6 months. He is an absent father. Our daughter is 4. I am married with a son that is 10 months olds. Plus expecting a new baby in June. She believes that my husband is her dad. Which he is because she is all she has ever known since she was 1. But my husband ask me "are you going to tell her about her real father?" I responded "yes. But idk when yet". So I have a few questions. So when is a good time or AGE to tell my daughter. I'm nervous on how she will feel about it. Also, Has anyone ever been told by their parents about their real parent? How did you feel about it? Comment please!!