I wonder if something has happened in his life that disrupted his routine? A divorce? A change of schools? A new house? A new sibling? First, I think 4 is a little young to be diagnosed with ADHD. Second, the sudden onset of symptoms is strange. It sounds more like an adjustment issue and that he is acting out because of a change in his life than ADHD. I would check with other people to see if they've noticed changes- does he just act out at home? How is he at preschool? Friends' houses? Etc.