[Theohazard]

As far as I know, the Gen 4 guide rod and ejector issues were fixed well over a year ago. I don't know about the numbers on the guide rod, but all Gen 4s come with a medium backstrap and a large backstrap (and the most recent ones come with two more backstraps with beavertails). The small backstrap is a permanent part of the frame; if you want the grip to be the smallest of the three options just don't put any of the extra removable backstraps on it. So with no backstraps on it's small, then you can add either a medium or a large backstrap to give you three total sizes.

If you got a new Gen 4 with no backstraps in the box, something is wrong. Is it possible you actually have a Gen 3? Gen 4s have "Gen 4" marked on the slide.