Actually, yes, on purpose. Ten miles from my house, a 4 year old got angry with a babysitter because the babysitter wouldn't let him/her do something. The toddler went to the closet, opened it, grabbed an unloaded SHOTGUN, picked up the shells and LOADED IT, then proceeded to walk over to the babysitter and shoot him to death.
Just curious... Are you referring to this incident that occurred in Jackson, Ohio? In that case, there were no fatalities. Just some minor pellet wounds for the babysitter and a bystander.
As for the child, he definitely showed that he had fairly advanced fine motor skills and was able to display excellent memory recall (either through learning by repetition or by watching adults). However, he wasn't able to distinguish between a real firearm and a toy gun, and probably didn't appreciate the difference (or consequences). He also didn't exhibit much planning ability. The whole thing was pretty spur-of-the-moment.
This is fairly normal development for kids in the four-year old range. It's right at that transitory area between two of Piaget's stages: pre-operational vs. concrete operations. At that age, kids are already physically able to do some fairly complex things, but are only beginning to understand the consequences of negative actions and concepts. I think most people have noticed that this is around the age when kids begin to actively lie, bully, cheat, etc. So, it's hard to say what his motive was, other than to express his disapproval with the babysitter.
(Note: I'm trying not to talk out of my ass. The above was written after consulting my spousal unit; she has an MA in clinical child psychology and works with developmentally-challenged kids.)
By the way, I'm not sure that I believe the incident went down exactly as reported. According to articles that I read on other news sites, there was a fairly large group of kids in the mobile home, ranging from infants up to late-teens. The alternative theory is that the teens were goofing around with the shotgun when it accidentally went off, then laying the blame on the four-year old.