The gen 4 is slightly smaller with a different texture. The issues do "seem" to be resolved with the gen 4's, but final results are still TBD. Buying a pre 2010 gen 3 glock is the safest bet if you are at all worried about reliability issues.
Stock, I prefer the gen 4 frame as it is slightly slimmer and rougher texture. That being said, I prefer my glocks in gen 3 (pre 2010 production), finger grooves and hump removed, and stippled frame, I've never done this to a 26/27 though.
That's the extraction problem I mentioned above. It has been resolved and there are new extractor part numbers now that will fix it. Fixed for free, of course if you call the customer service line. Or an armorer at a GSSF event will do it for you for free while you wait. If you're looking at guns to purchase, you can tell the extractors apart by sight if you know how, but I never learned because mine's a Gen2 and I'm not in the market for a Glock right now. Check Glocktalk and you'll quickly find more info....my biggest was, at least with the g19 in a gen4 it didnt really "shoot" out the brass when it ejected, it kinda just almost "rolled" out, if you will, there are a few videos shot and slowed down so you can truly see, and a few times the brass comes damn close to going back into the chamber before it closes to load the next round in. Just my 2 cents.
That's the extraction problem I mentioned above. It has been resolved and there are new extractor part numbers now that will fix it. Fixed for free, of course if you call the customer service line. Or an armorer at a GSSF event will do it for you for free while you wait. If you're looking at guns to purchase, you can tell the extractors apart by sight if you know how, but I never learned because mine's a Gen2 and I'm not in the market for a Glock right now. Check Glocktalk and you'll quickly find more info.