"Your son has autism"
Is that what they said? Was anyone with you? Were you expecting it?
I am so sorry that you feel devastated, but it's okay to feel that and other things, and please believe me, you will be okay too, and feel better soon. Take a deep breath, this is not the end, it is just a new beginning, in a new place.
Your son is the same adorable little boy that he was last week. Before his diagnosis. He has not changed. His diagnosis does not change him, it's just a label that explains the way he is. It will give him access to help and services that he may need to become the wonderful little person that he is destined to be. When you are ready, his diagnosis means that you can find support, help, advice and understanding. He's only four, he has a whole wonderful childhood ahead to enjoy and experience, even though it may be different to the one you were expecting.