My DS is 4 and ADHD has been mentioned
Mentioned by whom? Was it someone with the medical qualifications and capability to diagnose ADHD? Or another parent at school or a teacher or a relative, or someone in the corner shop?
Generally, ADHD isn't diagnosed until kids are much older than 4, because until then they could be just energetic little boys. However, at a certain age, the differences start to show as considerable, that's when diagnosis is appropriate.
I'm concerned we label behaviours a bit too freely
Sigh, there's always one, surprised we got to five pages in before the twatty "label" comment.
Labels are things you stick on suitcases.
Professional medical diagnoses of autism and its co-morbids and other conditions are done by a team of medically qualified professionals. It can take years, sometimes several years to obtain a full diagnosis from the NHS.
And before you insinuate that people can 'buy' a diagnosis by going private, no, they cannot.