I would do it. Not only are you getting a gen 4 but you're getting a 40. I have a 26 and I would love to trade for a 27. It all depends on you though. If you prefer the 27 then do it. Don't let the fact that it's new be your sole reason for choosing to do it.
Actually I have the gen 4 he has the gen 3, I would be increasing to a 40 which I have been looking into anyways? But, I'm not sure if it's worth it giving up a gun that I know is extremely accurate I'm kinda torn here!