I'd guess that D&M, like Microsoft, underestimated the demand for those models. After all, if you can't find them now, there certainly won't any available at close-out prices later.
A newer, better(?) device is always about to be announced. Get what you want when you can afford it. If you expect a lower price later, remind yourself that the additional money that you spend now pays for the additional time you'll be enjoying it. In this case, it'll be at least several months.
FWIW, a replacement for the 4520 is not likely to be announced until summer of 2014 at the earliest, but I'm guessing that's a little out of your price range.
Marantz SR7009 avr + MM9000 amp --> Atmos 7.1.4
Fronts=NHT 2.9+AC2, FH+TM=DefTech PM1000, LCR+TM amped