I think that it is possible for a Glock to fire while slightly out of battery. I believe that Glock went through the trouble of redesigning the recoil spring in Gen 4 to avoid that. The Gen 4 Glocks appear to be a bit more positive in their lock up as the barrel slides in the slide.
If the number of KBs goes down on Gen 4 it could prove my point.
The new chambers in 40 S&W also appear to be tighter than some earlier versions.