I just learned a little more about the beavertail backstrap change on the Gen 4s. I work at an LGS and a while ago I noticed they started adding the beavertail backstraps to the boxes, but until yesterday I never noticed the notch on the frame meant to secure the beavertail from flopping around.
I went through all of our Glock backstock; in Gen 4s we have a bunch of 17s, 19s, 22s, and 26s and a few 23s. Most of them have the beavertail backstraps in the box. But, with the exception of just a few, even the ones with no beavertail still have the new notch for accepting the beavertail. Even a bunch of our 26s have the notch, but none are packaged with the beavertail.
Our Glock rep says they aren't currently selling the bevertails separately, and they have no plans to ship them with the subcompacts. But it looks like they're keeping their options open by putting the notches on all newer Gen 4s, even the subcompacts.
To bad about the flop. I really need one on my G22 and I don't have the notch. I tried the GFA and didn't care for it.
I just tried the beavertail on two Gen 4 19s, one with the notch and one without. The beavertail is locked in place by the one with the notch; it moves around a little on the one without it. You might still try it out on yours if you can get ahold of a beavertail; the flop might not be a deal-breaker for you.