Hey Dennis,
I think you are correct unless the specification is a misprint which I doubt. So that re-opens the questions of "could 4 x 4 GB be installed" to that I'll have to say I just don't know.
I downloaded the manual
http://manuals.info.apple.com/enUS/iMac_Late2009UG.pdf which states
"Your iMac has four memory slots. Two are filled with at least a 2 GB memory module. You can install additional memory in the free slots or replace the memory in the filled slots with 2 GB or 4 GB memory modules, for up to 16 GB of memory."
so it appears somewhere Apple has made a goof.
Thanks for pointing that out, I was reading an article earlier from "http://gizmodo.com/5385841/apple-imac-hands-on" which implied the 21.5 only had 2 RAM slots. Maybe on my next trip to the Apple store I'll find out.
Regards,
Roger