Well at this age, developmentally, they have "fears."
And their emotions, are not even fully developed yet.
So their sense of articulate comprehension of appearances or what they mean, in conjunction with social aptitude.... and empathy, are not yet, fully developed as well.
And their 'imaginations' and cognition, is still forming as well.
They don't make sense of the world, like we Adults do.
Your Daughter is 4.
The aptitude and ability for 'deductive reasoning' is not fully formed at this age either.
Don't force her.
Just comfort her.
But in time, teach her about people. Knowledge like this is gained, in stages.
Maybe if she sees YOU... interacting with this boy & the Dad, and making it no big deal, she will, via observing you, learn from that. In time.
Just let the Dad know, your girl is only 4 and she is not yet, aware.
In Kindergarten, my daughter had an Autistic classmate. He had an Aide with him in class. She asked me what is Autistic. I explained to her, that he is a child just like her, but he learns & understands differently. So the Aide helps him. But all children are special. And everyone is different. My daughter then was not 'afraid' of him. In fact, she was the only student in class, that made friends with him. She tried to make him feel comfortable etc. and would talk to him. His Mom was so grateful. To this day, his Mom remembers my Daughter. And she is now 8 years old. She still looks out for him too. Around campus.
Kids in time, and per our teaching them, will learn all sorts of social constructs and that people, are varying.
Also socially, a 4 year old, is still learning. They are not yet fully mastered in social nuances. So don't expect her, to 'play' with him or interact.
Some kids, take time to warm up to people. Even if they are the same as them. My daughter is like that. But she is aware of people. Because she is taught.
If your daughter gets really upset and distraught over it, then is there a kiddie classroom she can go to? Many churches have that for the kids.
But I would not force her to befriend this boy.
Special needs or not, kids this age get freaked out about the looks of people. My son, when younger, would freak out when he saw my cousin. Because, something about his "hair" made him scared. This cousin, has real bushwhacked hair... like a cave-man.
My Cousin, just treated it like no big deal. We didn't make a big deal of it, either. My Son now really loves this cousin.
Also, my son was already 4 years old last Christmas... he STILL didn't even want to go anywhere near the "Santa" at the Mall. He was scared. No biggie. I didn't force him to take photos with Santa. He would only stay like 15 feet away from Santa. That is the closest he would go.
So now, I have a photo of my son "with" Santa... but standing like 15 feet away. Its cute!
all the best,
Susan