I have twins - one of whom is "normal" and one of who was born missing her corpus callosum (the part of your brain that's a bridge that connects your left & right brain - can cause autistic like symptoms) & is deaf. We started dealing with issues when they were 3 and are still dealing with some from time to time. We had to reassure her that by wearing the same shirts she couldn't 'get' what her sister had, we've had to try to explain that her sister is missing part of her brain and that is why she can't do things like she can. We've agonized over potty training as we are still working with one but the "normal" one was totally frustrated that she had to pee/poop in the potty but her sister still got to wear a diaper...lots of fun things like that! Now, the girls are 7 and we have a 3 yr old boy that we are starting the process all over with him.
The only thing I can say is continue to be as honest and up front as you can without overloading him. They don't have to know every single detail right now. Keep reassuring him that he is fine and can't catch what his brother has.
We are Christians and believe that God has a reason for bringing each child into our lives and we have expressed that to our children as well. We have all learned sooooo much from her - and our other two - already! The "normal" sibling's prayers for her sister are the sweetest prayers I've ever heard! So for us, leaning on God for the answers and the patience when we don't get the answers we want is what keeps us going. He brings us JOY in the midst of the trials & frustrations of having a child with special needs.
I can already see the compassion that having a differently abled sibling has added to her sister's life. We are careful to make sure that she gets alone time with us as we spend so much time going to therapy for the other one. We have let her know that it's ok to feel bad about having a sibling that's different, that she might be embarrassed or even made fun of because of her sibling. But we've also made sure she knows that is her sister for life, that she will always love her, and that she needs to help others around her understand a few things about her sibling - how to adjust playing so that she can play too, that it's not ok to call people names, etc.
I know I'm rambling around...you can also start looking at books that are written by siblings of differently abled kids and talk with your 4 yr old about those - read them first - one that I got was awful!!! The author was totally depressed and without hope. She basically said that there is nothing we as parents can do to prevent the "normal" kid from feeling like a hated stepchild for life...She didn't understand that most of her issues were really just sibling issues not I'm normal & my brother isn't issues...
There are sibling support groups you can locate in your state. Conferences to attend to gain support and be able to talk to others who are in your same boat. I know 2 people that have twins where one is "normal' and the other isn't. One is open & willing to talk and we have both grown so much from the experience. The other lady won't talk about it at all. She is sad and withdrawn and doesn't get out with her kids. So, please find some other people that can relate to what you are going thru and talk with them. And if you are a part of a church - usually you can find some good support there even if they don't totally understand the details!
Sorry for being all over the place, I didn't get much sleep the last 3 nights! Hope something I wrote helped!