Somewhere on the internet, I saw a Glock order form for LEO's and as I recall, there was no price difference for RTF2 vs. Gen4. But that was for LEO's. I don't know if there is a difference in dealer cost on the retail side of the sales spectrum between RTF2 and Gen4. I'm planning to drop by my local range tomorrow and I'll see if I can find out if they are charging extra for the RTF2 or Gen4 versus "straight" 3rd generation models.
I wouldn't be surprised if some dealers charge a bit of a premium for a Gen4 due to its relative newness, especially as new models enter the pipeline in that trim.