I have on order a normal length 10mm->.40 KKM conversion barrel, and had that exact same question: Would it work in a Gen4 G20? I called KKM and the nice guy I talked to on the phone there told me that Yes, the .40 conversion barrel they have will work fine in a Gen4 G20. I did not ask about a 6" barrel as that's not what I was looking for.
I'd say give them a call to confirm, but, at least the stock length .40 conversion should be a drop in.
As far as the mags, what I've read is that the older mags will work OK, but I thought there was some qualifier attached to that like, the ambi mag release or something won't work in left hand mode. I'd say to research that some more, or, contact Glock.
EDIT: As far as getting to where I found the Gen4 10mm, just go to the main Glock US page, and click on Products. Then, on the next page you end up at (the Product page), click on Hunting. Should load right up for you.
Last edited by chucky2; 01-31-2013 at 10:04..