Are the Gen 4's on the shelves all up to date with these upgrades by now, or is there a some way to tell before purchase?
I can't say for sure, since I don't feel like digging around in my closet, but on the box a production date is usually in with the other model information. You could ask to see its box. Otherwise, if you can go to a LGS that does a lot of business and you're at least more likely to get something from a newer batch than something they've had for awhile.
There is also a number on the ejector(IIRC) which could be used to determine if it's an updated one. However IIRC that number cannot be seen with with just a field strip.
Hopefully someone who knows the details of this can comment further.