I think HexHead may be onto something with his diagnosis. I dont have any Gen 4 Glocks, but I do have a number of Gen 3s, including three 26's.
One thing is for sure, Glocks do like hot ammo, and do perform better with it. Glock even sent a factory letter around back in the late 80's recommending it, due to issues with the then current factory 9mm. They were recommending the new at the time, UZI "black tip" carbine ammo, of all things. That was the only stuff that actually ever started fire forming the brass into the flutes of my MP5!.
My experience has been, that ammo, and especially reloads, may very well be the main cause of the BTF issue. This seems to be even more of an issue with heavily reloaded brass.
As far as reloads, and power issues, I learned early on back in the 80's, that my reloads that worked fine in my P38's, HP's, HK P7M13, and a couple of SMG's, would not cycle reliably in my then new Glocks. I had to move the load up towards the upper end of the load data to get reliable function. My 9mm reloads today, still reflect that too.
Now, onto the brass issue. Right now, Im cycling through about 6000 rounds of 9mm brass, that I shoot and load weekly. I shoot my brass to failure, and about half of that 6000 is starting to get to the point of having issues. The two main indicators are, split necks, erratic extraction/ejection, and increasing BTF issues.
A couple of years back, when all you heard was about Glocks having BTF issues, and it was extractor related, I started to have the problem, and on an increasing level. I tried switching out the extractor, and its related parts, and continued to have the problem.
Since 99.9% of what I shoot, are my reloads, I began to notice more and more split necks, and decided to scrap that lot of brass, and picked up some more, once fired brass to replace it. Lo and behold, when I shot some of that new lot of brass, I didnt get one round of brass to the face. I got to thinking about it, and tried a couple of boxes of WWB I had on hand, and had the same experience, not one round to the face.
Switched back to an older lot of my reloads, and the random BTF started to occur again.
This has continued ever since, and Im more and more convinced, the older and more worn out the brass is, and more chewed up the rims get, from constant extraction, the more the issue occurs. Switch to factory, or newer reloaded brass, and the issue stops.
After kind of coming to that conclusion, I put the old extractor back in the gun, and tried it with the new brass, and factory ammo, and guess what? Yup, no BTF. That same extractor is still in the gun today, and has 25,000 to 30,000 more rounds through it.
I do still get BTF now and then, the occasional stove pipes, even no extraction with a double feed once in a while, when using the older lots of my reloads. Just looking closely at the cases, is usually all it takes to understand why.
On the plus side, it lets you more realistically practice your IAD's on unplanned and random malfunctions.
Now, with factory ammo, I can see if youre using ammo that is not loaded to what Glock considers "spec", power wise, you might well have issues, and issues that other guns dont seem to have with the same ammo. Just like my early 9mm reloads, werent hot enough to cycle my early Glocks.
By no means am I saying that maybe Glock didnt/doesnt have some issues with the Gen 4's, it does seem kind of odd, all of a sudden they did, or seemed to. Its really kind of hard to know, just who was doing what, and what exactly they were doing/shooting too, so there may well be other things going on as well. If you are experiencing problems, a quick, cheap (well, maybe not cheap
) check, would be to try hotter ammo and see what happens.