As the owner a generation 3 G26, I can say that the changes they made for the Gen4 seem pretty much useless and unwarranted. Judging by the picture, the recoil spring / guide rod is the same as the one in my gen3. The only noticible differences are the removable backstraps, the rough textured frame (RTF), and the enlarged magazine release.
I definitely wont be upgrading, but I see no reason you wouldn’t want the gen4 (if you are in the market for a subcompact Glock) if it is being offered at the same price as the gen3 models.
I had my fingers crossed that the changes would be drastic on the subcompact models. I guess you don’t mess with “Perfection” though right? heh :P