The slip on grips are nice, like this one, just be aware its going to act as a palm swell, so if you're already reaching for the trigger, it may end up too large. I'd say most of the time they're beneficial because they'll actually help force your finger into a better position with regards to how you index the trigger. Plus Hogue grips just feel amazing. You'd know if it were a Gen4 as it's got that marketing tool plastered on the slide, and just looks a little different. Chances are you'd be better off with a Gen 3 anyway, seeing how slow Glock has been to acknowledge and then fix the fourth generations very apparent issues.
I have that slip on grip on my Gen 2. I like it on the 2. It's not too big for my size 10 hand but it's close. Not sure it would be needed on the 3 but I've never shot a G19 Gen 3. If I was going to get another 19 it would be a Gen 3 I guess. My Gen 2 looks like it will last forever though