I'd have to say this..
4 years old? Stick to "guns are bad"
When I was 4, Girls were bad, Artichokes were bad, all kinds of stuff was bad. Strangely, Twinkies were good.
Things have changed alot since then, and I havent had a Twinkie in years!
I would teach "Don't Touch and tell an adult", and don't even push the gun issue on the kid.
When he's around 10, buy him a cheap bb gun..and go have some fun with it plinking soda cans for a few days. Then convince him to shoot himself in the foot with it (I didn't need any coaxing myself). Instant respect is then gained, and you should go out and get him a high quality pellet rifle. Test the waters and ask him to shoot himself in the foot with it (BUT DON"T LET HIM). If he says "No Way!", then you've done well.
Now at 10, he has his own rifle, he enjoys it as a fun "good" thing, but has respect for it's power.
That's how I came up anyway.
At 15 I was given a 38spl target pistol from my step dad that he let me shoot many times on the range, and at 18 I had a Mini-14.
So far, so good. I've stayed out of trouble, and now enjoy firearms as a positive part of my life, and I still control my firearms..they never control me.
Just my thoughts...hope it helps.
Salute to you GNR for caring.
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Same Shot, Different Day