I'm not sure this means what everybody thinks it means. Most books have a short shelf-life. They're released, there's a surge of sales that drops off fairly quickly, then they're remaindered (put in the bargain bin to get rid of excess inventory). Books that aren't sold are returned to the publisher for a refund, or they're destroyed and their covers returned for a refund. And then that's it. The book isn't reprinted. It's done.
(See
How Thor Power Hammered Publishing for an interesting article--no, really!--about the finances of the publishing industry and how tax laws encourage remaindering.)
So it doesn't make sense to say "4e has hit the bargain bins."
Which 4e books have hit the bargain bins? Are they hardcovers that were released more than 6-12 months ago? If so, this isn't a surprise, and probably doesn't have anything to do with 5e at all.
Even "core" 4e books like PHB1 and DMG1 being remaindered doesn't mean anything, because those books have been replaced by the Essentials line.
Now, if we were seeing Essentials being remaindered, that would tell us something. But we might not ever see that, because Essentials books are paperbacks and they could just get destroyed instead.
Discounts on Amazon don't tell us much, either. The publisher gets 50% of the cover price and the retailer gets the other 50%. The retailer will often discount the cover price--cutting into their 50%--in order to sell more. The more popular the book, the more likely it is to be discounted, because the retailer knows people will comparison shop popular books, and the retailer wants to give the impression of having the best overall prices. So, ironically, a book with a nice 32% discount on Amazon (which is what Rules Compendium currently has on the US site) is probably selling pretty well. My book spikes up to 45% off at the beginning of every semester, because it's used as a college textbook and Amazon competes with college bookstores.
tl;dr: Remaindering older hardbacks is normal and doesn't tell us anything about the popularity of 4e or the significance of 5e.
(Note: I'm talking about mass-market bookstores, not hobby shops. Also, although I have some experience due to authoring a published technical book, I'm hardly an expert on the publishing industry and might have gotten some of this wrong.)