As a threat after school if my 4 year old DS has had a good day, we sometimes go to a local pound shop and let him pick any toy. That's the good thing about this age a £1 toy does the trick 
I've noticed that the store keeps having more and more toy guns. DS is fascinated by them and has been wanting me to buy him one, specially today as he could see that other children were buying them. I told him that guns are bad and hurt people, and that although those are only toys, that it's not good to pretend to do something that can seriously hurt people. I believe he understood because he stopped asking and picked another toy.
I'm I being unreasonable? Will this just increase his curiosity?