I don't think four year old individuals, especially other people's children will on average grasp reality firmly enough yet.
What is the difference between the TV show they watched where the person they wish to playfully emulate shoots someone, and real life at the range? The video game where they shoot at people?
I have had children that age do some stupid things before. Like one hit me in the leg with a baseball bat totally unprovoked thinking it was funny.
Five or six would be minimum ages to begin safety in my opinion, and then not actually given complete independent control of a firearm.
You normally begin teaching with a long gun for a few reasons.
The biggest one is the ability to correct muzzle control. If they begin to drift off target with a long gun you can grab it and redirect it, or take it away. It also takes more intentional movement to change the aim of a long gun.
A pistol can be pointing 90 degrees in a different direction before you even see it coming even as you stand behind them within arms reach.
I can recall even new shooting adults gesturing when they spoke with a pistol in thier hands and sweeping 180 degrees.
I would begin with something that will not pose a lethal threat and see how they handle safety instructions before putting a firearm in thier hands. Like a low power airgun.
If you must use a firearm, a .22 long gun with you in range to take it away if necessary is much more appropriate for a new shooter.
Consider that such a child is not even legally responsible for thier actions. Someone that age who shot someone dead would not even spend some time in juvenile detention. They don't even get the legal version of a time out or a slap on the wrist.
Consider this:
http://en.wikipedia.org/wiki/Defense_of_infancy#Ages_of_criminal_responsibility_by_country
Until the age of 6 at a minimum in North Carolina, 7 in most of the nation, and between 7-12 in various states, and 10 at the federal level, a person is not responsible for thier actions. That means a 4-6 year old could decide to murder several people at the range and suffer no punishment in virtually every state. Maybe thinking it would be fun to just shoot people like on X video game?
A firm grasp of reality and consequences, or even a grasp on life and death does not exist in most children by that age.
I would be entirely for the adult responsible suffering the full punishment as if they had fire the shots themselves if they provide someone under the age of criminal responsibility with a lethal weapon that is misused.
The mentality of an individual at such young age is highly questionable, especially of a child you have not raised yourself.