Sibley has extensive discussion on separating Hoary and Common on his website:
http://www.sibleyguides.com/bird-info/common-redpoll-and-hoary-redpoll/
I don't have much experience with Hoarys, except for watching a few in a large group of Commons at a feeder in New Hampshire a few winters ago so take my comments with a grain of salt. That said, I agree that the pictures don't scream "Hoary." While the picture quality isn't good enough to make out fine detail, in https://dl.dropbox.com/u/67275536/P1020099.JPG the one bird showing its undertail coverts doesn't look as unmarked as I'd like to see before calling a bird a Hoary. My last thought is that I'd be surprised if Hoarys flocked together such that you would get four Hoarys at a feeder to the exclusion of any Commons.
Max Wilson
Kensington, MD
mercretas AT hotmail DOT com
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