A G19 was the first pistol I bought 14 years ago this month. It was a Gen 1. Since then I've owned 2 Gen 3s and 1 Gen 4. I have shoot thousands of rounds through the 19s and can't think of a single malfunction. I usually shoot the cheap Winchester White Box for training and Federal HST for defensive carry. The only thing I've done to my G19s is swap out the sights. I feel that is a must and I prefer Trijicon HD in orange. I've considered undercutting the trigger guard but haven't done it yet.
Between the Gen 4 and Gen 3. I can tell you that I will only own the Gen 3 for CCW. I tried the Gen 4 and very quickly found that the texturing is too aggressive for Inside the Waistband Carry. It is very uncomfortable when rubbing against your side. I carry a Gen 3 either at work or in a CCW role all day long. I have no issue with it and they have yet to let me down. I usually clean it every few months when I start to notice a build up of deodorant and lint.
Hope that helps.