I think I'll wait for this story to appear in an actual news source, because there are too many oddities for me to believe it's real.
Firstly, a four year old able to accurately headshot someone? Guns are heavy, and firing one means a small explosion is going off on the gun in your hand(s), making it difficult for an untrained ADULT to hit someone in the TORSO. Let alone a deliberate freaking headshot by a four year old.
Secondly, even if video games are somehow magically at fault, the kid should not have been exposed to them. Games with pronounced amounts of even semi-realistic gunplay are at least rated TEEN (or equivalent rating in other countries). The most I'd maybe let a four year old see is little big planet. Games have a system in place designed to inform parents that maybe their kid shouldn't be playing Gun Game: Shoot All the Things until they are psychologically able to tell that this is not a good thing to do in real life. If this four year old had been exposed to large amounts of ANY violent media, then I could see this being a "monkey see monkey do" where the kid sees the gun, is upset at dad, and shoots. That is not the industries fault, that is the parent's fault for letting a four year old be exposed to copious amounts of violence. And, y'know, for leaving a freaking LOADED GUN around your kid.
Thirdly, I'm going to reiterate Smeagol150's point, why was a round chambered in this gun? For this to happen either this four year old knew how to turn off the safety and chamber a round (which a kid that age may not be able to do, I gather it requires a bit of effort even from an adult). That also means I'm pretty sure he'd been taught by more than video games, I've played a lot of shooters and I have no clue where a safety would be on a gun. So either the kid did this, or the gun was lying around with no safety and a round ready to fire. That makes it not an issue of video game violence, but an issue of total absence of gun safety in a home.
TL;DR, this doesn't add up, and even if it is true, I really doubt you can blame video games for it. Not that it will stop Fox News from trying once they hear about it.