I would think airbags only help the neck if a 3 point seat belt is worn.?.? I mean, the belt should stop the forward travel of the body. But the head is unsupported and keeps on moving. So, in most cases, I would assume there isn't much difference between a 3 and 4 point. Though, the force would be spread across the body more evenly w/ a 4 point... I'm just spit ballin' here...