Yeah, Gen 4 Glocks are starting to come with four backstraps now: Two of each size, one regular and one with the beavertail. Unfortunately, the beavertailed ones don't fit well onto old Gen 4s.
The new Gen 4s that ship with the beavertail backstraps also have a small square notch at the very top of the frame at the back of the slide. The top of the beavertail fits into this notch; if you put a beavertail grip on an older Gen 4 without the notch, your backstrap wil flop around.
I know this because my friend pointed out that notch on the new Gen 4s to me. He got ahold of a beavertail backstrap to put on his older Gen 4 19 that doesn't have that notch, and he said it flopped around so much he just took it off.
0331: "Accuracy by volume."