Shot both extensively. Much prefer the Gen 4, for the grip texture, mag release, and option for factory beavertail backstrap. I personally think the grip texture alone is reason enough to favor Gen 4 guns, and it's especially important with the G26.
I've never noted any type of practical, quantifiable difference between the Gen 3 and Gen 4 trigger bars in timed shooting, even though the Gen 3 does indeed 'feel' nicer in dry fire.
I've carried a G17 IWB, for probably 95% of the last 10 years. The other 5% is G26 territory. In real-deal non-permissive environments, there's nothing even close to the utility of a 26. IWB in kydex with a holster that sports wide, stiff attachment points, the thing is honest-to-god 100% invisible in a T-shirt. I never feel the need to surreptitiously 'check' that my shirt hasn't bunched around the grip of a 26, like I do with a 17. And I agree that there is a noticeable improvement in concealment when compared with a 19.
I also echo the sentiment that the 26 is nicer to carry IWB in a holster designed for the larger guns. The longer holsters offer more stability and less shifting on the belt. While I have two nice, high end G26 kydex holsters, I've found I prefer just carrying it in my old G17 holster.
I *strongly* disagree with anybody that feels the G26 needs any kind of grip extension. I feel it's mostly novice shooters that think the dominant hand absolutely, positively needs all three fingers on a subcompact handgun's grip, though I do occasionally see accomplished shooters claiming they carry their G26 with grip extensions. Just fold your dominant pinky underneath the mag baseplate. It works. You're carrying a G26 for the concealability. If you really, really feel you can't live without G19 grip length, just carry a G19.
The overall awesome shootability of the G26 is just insane when compared with other guns of that size. I'm consistently impressed by how good it is every time I shoot it on a timer. It's remarkable that a gun that small can do 80% of what a G17 does, which we can probably all agree is the general standard of service pistol capability.