M.J.
In our son's case, they stopped when he started ADHD medication at age four (Ritalin). Medication changed everything. It's like giving glasses to those who can't see well. However, because it doesn't last all day even in long-acting forms, you still face challenges first thing in the morning and later in the day when it's out of the child's system. We have found that medication decreases the severity of it all the remainder of the day, but it still can be difficult. We haven't seen the severe meltdowns anymore, though.
If you haven't discovered ADDitude magazine, it's worth subscribing. I've found it incredibly helpful in navigating this misunderstood condition. And they have done articles in the past on ADHD in preschoolers, so check out their archives.
To the person asking what you see in a child who's diagnosed at age four, I can tell you from experience. Our son was so aggressive and unfocused he was kicked out of preschool, he couldn't sit for circle time, couldn't sit for any stretch of time, was unable to control his body and just moved constantly, would run from one wall of our house, across furniture and to the other wall of the house (literally bouncing off the walls, no matter what we did to try to stop him), was the Energizer Bunny ALL day -- not like normal kids (we have an active daughter who is nothing this extreme) who crash at some point. Time outs were impossible because he couldn't sit. He didn't respond to rewards or punishments no matter how consistent we were. It's off the charts stressful living that you can only understand if you've had a child with ADHD. It's so far beyond "oh, he's just an active boy, he'll grow out of it." It's that to the 100th degree.