I carry a Gen 3 26 in a kydex AIWB,. I would recommend the Gen 3, the smoother grip is more comfortable against your waist AIWB, especially if your holster doesn't have a big leather shield, If you carry in something like a M TAC, no difference. The 26 gen 3 and gen 4 use the same dual recoil spring setup. Don't have a gen 4 26, but will occassionally carry a gen 4 17, AIWB, the extra texture is tolerable against the skin, bearable but you know its there.
I have a Raven Concealment Phantom holster for mine and have the inside and outside waistband loops. I like the inside carry better but it is uncomfortable unless I have a shirt tucked in my pants. No problem in cool weather but I'm looking for another solution for the summer when I don't want to wear a jacket or two shirts.