Hi, I saw pictures on the internet that have led me to believe that my son also has ACC. He will also be 4 yrs old soon, too. There's a great website (makingcontact.org) that lists 16 others describing their stories re: this condition (do a search for 'Aplasia Cutis').
Did a Dr. make the diagnosis for your daughter or did you do your own internet research too?
I'm a 16 year old girl with the condition. Theres not much you can do about it. I have it in 2 or 3 small patches on my scalp. They do not affect me except for a painful sensation when touched.
Where is it on your daughter?