Update:
For any of you purchasing a Generation 4 Glock, I can now confirm that the following Pearce Grip Frame Insert (sometimes called a "backstrap plug") fits the Gen4 G17:
[size=10]GLOCK Mid and Full Size model Grip Frame Insert. This unit installs in the bottom rear cavity of the frame utilizing the lanyard hole for retention. Fits Glock Models 17, 18, 19, 20, 21, 22, 23, 24, 25, 31, 32, 34, 35, 37, 38
Part No. PG-GFI[/size]
I had been told by one who claimed to know that the Gen4 would not accept some of the after-market backstrap plugs made for earlier generation models. However, the above-described Pearce Grip Frame Insert, one of which I just received by mail, snapped perfectly into my wife's Gen4 G17.
http://www.pearcegrip.com/glock_pricing.htm