Telling a 4 year old about his Asperger's diagnosis... Any advice?

Hi, we recently had an Asperger's Syndrome diagnosis for my four year old.  He is very self aware about his behaviour.  We haven't told him about his diagnosis properly. I bought a picture book about Asperger's and he identifies with the boy in it. He starts school in September and I am wondering how and when we should tell him properly about his autism.  Has anyone any suggestions or experiences which they can share.  I have looked at other threads but they all appear to be about older children.