We told our 6 year old that his little brother's imagination doesn't work and it's hard for his brain to tell his body to do some things, like talk. We said that the teachers at his preschool are trying to help his brain learn how to tell his body to do those things. He seemed to get it :)

I really like the book "All Cats Have Asperger's Syndrome". My DD has HFA rather than Asperger's but I told my older kids that it was similar just with a speech & language delay.

For a 4 year old it might be enough to just explain everyone is different in their own way and we should all try to accept those that are different. I'm not sure a 4 year old would comprehend what ASD is. Check the library for books that you could read to her. Don't bring Jeremy is an advanced read (lengthy too) but a great message about siblings being together and looking out for one another.
I have 4 children and never labeled my 14 year old to any of them. It's who is and how he will be :)

First off mostly my 14 yr old explained to my 8 yr old about my non- verbal 11yr old with autism. Kids know how to talk to other kids. And when I explain to my 8 yr old he is actually more clued in than I thought he was he pretty much explained it to me. I often(no matter what he askes me) ask him what he thinks? I try to encourage him to have confidence in his own ability to figure things out and to not be afraid to ask questions when hes not sure about something. He asked me why Dezi can't talk? I asked him what does he think? & he replied: Because sometime people havent learned to speak yet! I told him he was right and then I explained what autism was in simple term. And that dezi need more time/training to speak. And maybe he wont ever speak in the way we hope but you can see that dezi is happy and will live a great life. As far as other kids understanding I tell their parents and the parents often explain it to their kids. But if a kid askes me a question about dezi, I never get mad ir make them feel ashame for asking. I just answer their question and alway add that although he doesnt talk much he understands everything and he is a normal boy.

When you figure it out tell me... except a 3y old their brother has asd. We havnt really explained it to his 2y 4y and 5y old cousins either.