It is thoughtful of you to come here to ask for advice on this - you've already done more than any of my friends/family just by doing this!
First thing I'd say is that Crawling is right, getting a dx at 4yo is pretty standard. When a child is diagnosed has no bearing on future outcomes.
I'm assuming things will be quite extreme if they have been diagnosed this early in life?
No, not at all. Did you think the child was "severe" before the diagnosis? If not, then there's no reason to think they are just because of the age of dx. Your friend's child is as 'severe' as he/she appears to you. Treat him according to how much you think he responds to you or his skill level or understanding, not how you think a "child with autism" should be treated. There is no reason why you should suddenly change how you treat him just because of his dx - he is the same person he was before.
My ds was dx at 3.6. He is 'high functioning', at mainstream school, highly verbal, bright, chatty, communicative, has friends. In many ways he 'passes for normal' (until you notice that he constantly talks about the same thing
). People are always surprised he has ASD when I tell them. Age of dx is not an indication of future outlook.
My personal bugbear is people thinking that "children with autism" do x or y or z. People saying "they" do x yz "don't they".
Your friends child is an individual as much as any other child. Autism, imho, doesn't define a person - it is a condition that affects different areas to varying degrees.
Definitely keep up playdates. The most important thing any other mum can do for me, actually, is to suggest playdates. It is really important.
I'd try and steer away from comparisons. The development of a child with asd is just different from nt children - so being 'advanced' in some areas doesn't mean the same thing. Your friend probably seems OTT about comparisons because she's trying to reassure herself about her child, not because she is actually competitive iyswim.
But do point out good/nice things about the child, whether the characteristics are because of or nothing to do with the autism.
I haven't found any of my existing friends or family particularly helpful tbh in some cases completely the opposite prob because they are all scared of saying/doing the wrong thing, so none of them really talk about it!
Take her lead about how to talk etc to him - and ask her if there's anything she would especially like you to do (eg in my case I ask people not to respond if ds asks them how old they are/what door number they live at/what size their shoes are as these are current obsessions of his that we don't want to reinforce).
Point her in the direction of this board! 
Sorry for the essay.