While I'm of the opinion that "curing" ADHD at a young age might be a bad idea [1], ADHD is a genetic difference in the brain that is present from birth. It's not a personality thing. It's not something that happens to you later in life.

1: as someone that got diagnosed in adult age, I am now aware of my talents and shortcomings, and all the things I have struggled with. I keep reading of kids on Ritalin that grow to hate their parents for having put them on medication, quit it, and find themselves completely lost in life. They have no idea how dysfunctional they are. They've never had a chance to experience how debilitating it can be.

Give or take a few years. There are a number of subtle developmental differences between individuals. Not saying the doc is correct on diagnosing a 4 year old with ADHD, as some docs still like to jump the gun on that. However, so long as the diagnosis results in evidence-based treatments that do not involve drugs, it may still be beneficial. I say this as a 44 year old who was not diagnosed until 41, and that was after almost 10 years of trying to explain to docs what was happening to me, and the hell that it made my life. From my perspective, it becomes a question of which is worse; diagnosing and treating too early (sans drugs) or trying to fix a broken adult set in their ways?

The CDC collects statistics on ADHD diagnosis starting at 2 years[1], breaking out the 2-5y subgroup, presumably because that is before 1st grade. The condition is definitely recognizable at that age and behavioral therapy might be indicated.

1. https://www.cdc.gov/ncbddd/adhd/data.html