I just bought another G17 for $399. It is in great shape and came with the box and two 17rd mags. It even had the spent cases from test firing in there too. My question is how do I tell the difference between the four gens? I know the ones I had before were both gen 3s. The only changes I can see on this one are the cutout on the front of the magwell isn't there and the slide is a lighter black. Different finish maybe? Any help is appreciated.