I would also think ADHD would be across different settings... the behaviours you have said the parents see, "seem" more oppositional than attention deficit.... maybe it's ODD they are saying he has...? ODD can be seen at home more than at school in some children...

He is also only 4, so..... it's all a bit question mark ish anyway....

In my experience sometimes paediatrician's over prescribe medication and incorrectly diagnose disorders based on limited information - usually just a conversation with the parents.... however I guess we are never actually IN someone else's situation, and anything is possible....

there's nothing much you can do, as telling them you disagree with the diagnosis is probably only going to put them offside... and you will be more helpful if you are involved - as it's so important to for parents to develop as calm and clear parenting strategies as they can to support children with any behavioural difficulties.

It is frustrating to watch a child get diagnosed when it seems all wrong... hopefully it will help the parents put in place the strategies needed to support him - as for some people the 'label' is really helpful (even if it's not totally correct, it's a start I guess.... !)