You may have to increase the level of supervision. That is not to say that you aren't a responsible parent, but kids with ADHD (particularly those with extreme impulsivity) just need to be watched more than the average 4 yo.
When my DS was 4, he had a moderate amount of impulsivity (his is more a problem of focus, attention, and about two years behind in maturity) but he was only allowed to play out of my sight if he was in his bedroom, which had basically been baby-proofed. If he wanted to play outside, there had to be an adult with him. If no adult was available to be outside, then he had to stay indoors. If he wasn't in his room playing (which was on the first floor of a small house) then he had to be where I was.
Even with that amount of supervision, we had hook and eye locks place on all doors high enough that we had to reach all the way up to unlatch them. We had child proof latches on the cupboards and drawers.
I don't know if you'll be able to find a doctor who will prescribe meds when he's just 4 (maybe, maybe not), but take heart, they do grow out of it some. My DS is 12 yo now and I am comfortable leaving him home alone while I do the groceries or some other hour or less errand
I think that our sons are twins! My DS still does this, and I think that it's the most disturbing aspect of his ADHD. I worry about how he'll handle himself socially when he's an adult if he's still saying seemingly conversationally irrelevant things in front of others. He's gotten better, but he's had a tough time with his peers for his odd conversation skills.
I forgot to add that I insisted that my DS be assessed for autism/aspergers 3x from age 6-10, and every time the dx came back ADHD. I definitely know where you are coming from! I trust the dx that we received but his behaviour was so out of the ordinary that I wanted all bases covered as he matured and developed.
I know what you are going through. My son is now 8 (will be 9 in a few months) and he was diagnosed close to the end of the school year last year. I think I always knew he had this, I just didn't want to admit it. He struggled through Kindergarten, 1st and 2nd grades with behavior that I didn't understand. I made excuses for him and he was a good student, when he did his work. But the things he did, it's funny when I look back at it, but I was horrified at the time. Just in K alone, he hit at least 2 kids, he was destructive, he pulled his pants down in the cafeteria. The crazy thing was that I never saw this at home. He has never been destructive, aggressive or anything like that at home. He got a little better in 1st grade, but 2nd grade he was still struggling. I talked to his teacher who has a son who was a lot like my son. She asked me if I had his tested for ADHD and I hadn't. So I talked to his ped and after the testing sure enough that's what it was. He had gotten to the point of being in tears because he couldn't concentrate to finish his work. He really wanted to but he couldn't. The same with impulse control. He knew the rules, but he couldn't think through it to make the right choices. It was very frustrating to see. The thing is, at home, we've never had as much of an issue. He is supervised at all times. When he is in his room or upstairs, I check on him every 5 minutes. I have to keep that watchful eye on him. Out in public has always been touchy. He gets over-stimulated with lights and noises and people and it gets him wound up more than he would be normally. I can see this happening at school and spiraling because there are 25 kids and 1 teacher. I can help him through it because I only have him to focus on.
So on to meds. I have to admit I was not for them. I know people overmedicate their kids to keep them "under control". I didn't want to be one of those parents. I talked to his aftercare and they told me stories of kids becoming zombies. It's scary stuff. BUT, I had a kid in tears because he needed help. There is no ignoring that. I trust his ped and he would never push something he didn't think was needed. I also didn't want my really bright kid to loose interest in school because of this. So we decided on a trial. Our ped recommended a fast release drug that if taken in the morning would be out of the system by lunch. I agreed to this as the teacher as well as myself could see what he would be like on it vs not. It was like night and day. I can't tell you the relief it gave all who had been involved. He was not a different kid, but he started completing his work and could follow rules and was a lot happier with himself. It has been almost a year now and we are one an extended release med that lasts while he is at school. Granted, it's not in his system in the morning or at night. So here are a few suggestions that I use for my son. Always have a routine. It is key to helping us function. Bedtime is always the same time. Morning rituals are consistent and I help out with everything (I still pick out his clothes-too many choices are not good for him). Have clear and understandable rules. Follow through with consequences. You have to be firm and consistent, but at the same time know what their limitations are to work with them and not against them.
First of all, where did you get your diagnosis? My son went to a neuropsychologist who gave us an 8 page report of suggestions specifically geared toward my son. He has impulsive and hyperactive type ADHD and has poor executive skills, which are understanding, remembering, and following rules. By following the suggestions from his doctor we had behavior results similar to medication.
After about 2.5 years he took a major downturn, so we started meds. He had a bad reaction to the first drug he was on. He only took 2 pills. Because he was going to school I gave them the heads up so they let me know his behavior change. His doctor was on the ball. It was bad, but it was over with very quickly. He took a couple of weeks off and started a new med, which has been fantastic. He's even has his dosage dropped. He's taking as little as possible to help him to focus. He is completely frustrated when he's not medicated, because he can't focus. It makes a huge difference in his life.
And as far as side effects go, kids who have asthma are on steroids, which aren't great for a body. But they need it. And if your doctor prescribes a med for your son it will be because he needs it. It's scary, but remember why you're giving it to him if you do.
That stated - I am a firm believer in trying behavioral/environmental modifications first, so get a list of them from your son's doctor.
Finally, I wanted to let you know what my son's neuropsychologist thinks is the cause of his ADHD. There are other causes. But for my son, he thinks it's due to parts of his brain growing at different rates. Remember in middle school the boys with the huge schnozes who grew 6 inches and were handsome by the end of high school? Bodies don't grow at a constant rate. My son's doc thinks brains don't grow at the same rate, either. There is research to back it up, but it's beyond me. In a nutshell, ADHD can be a temporary thing that kids grow out of. Unfortunately, it happens when they need to concentrate and learn things they'll never have the chance to learn again without being delayed. For that reason parents need to do things differently for their kids so they can learn despite having their brains out of sync.
Study up on this stuff. The more you know the easier it will be for you to make a decision for your son. There are bad stories - follow up to see how they turn out. Look at the percentages of parents who do nothing and are as frustrated as you are, or make an active plan, and how much better their families' lives are. It's scary, but you have time to make an informed decision. And anything you do can be stopped and reversed at any time. Try everything and see what works.
Good luck with this!