Sorry the response is so late, but I thikn it wuold be best to explain it to her in a way where you tell her she is lucky because so many people have "chosen" to be in her life. You can say her real father was sick or something but his family is here to love you. It's hard to explain to ta 4 yr old sometimes. Depends on their maturity.

Good luck.

I understand your dilemma. My ex-husband disappeared as soon as our divorce was finalized. My little girl was two at the time. She is 3 and a half now, and has an obsession with people ‘coming back’. If her grandfather, for instance, goes to the store she just ‘switches off’. As soon as he’s back it looks like somebody switched a light on in her little face, she’ll hug him and repeat: ‘you came back, you came back’. We shower her with love and attention but that hurt just doesn’t seem to go away.
I know she will one day ask me why her dad is no longer with us, and I dread the day I’m going to have to tell her. For now I think lil blaze is right. Focus on people who WANT to be with her.