Hi!
I have Asperger's myself, but I don't have ADHD, but my father has ADHD, but doesn't have Asperger's.
Although I think there are many children who have comorbid AS and ADHD.
They may have symptoms of Asperger's, and ADHD as well.
I advice you to read about symptoms of ADHD and Asperger's, think about it and then go with your child to a specialist and discuss about which of these disorders your 4 year old son may hhave. It's good to take some literature with you, anyway. It might help.
You must be careful about a specialist knowledge, because some doctors don't even know for Asperger's, so I advice you to go to a specialist who has ever worked with Aspies.
There a problem shows, cuz Asperger Syndrome is very likely misdiagnosed somethimes, when it's allmost "new" syndrome, and many people have never heard for it yet.
The biggest difference between ADHD and Aspie child is that ADHD child can never concentrate well, even if he is very interested in that thing or subject. Despite of that, an Asperger's child usually can't concentrate, but when the thing he does is interesting he might concentrate well.
Read some literature about differences between the two, and check out which signs your child really shows.
Don't allow your child to be misdiagnosed, it's very important!
Hope that helps ;-)
All good,
Aspie