A four year old probably doesn't have the capacity to understand death or the concept of murder... but it does have the capacity to know that some things hurt other people. The discussion above was more about the capability of the child to use the firearm, not the reasoning behind it. The child in question keeps asking where his brother is, not realising that he may well have killed him.

Murder is usually classed as 'with malice aforethought', if the child were going to be charged with any crime, it could only be manslaughter.

LTS