So far we only know that 4E sold better than expected.
We know more than that.
We know that at least the initial print run for 4e was 50% larger than the print-run for 3.5, which was larger than the 3e print-run by an unspecified amount. That means if 3.5 was 100,000 copies, then 4e was 150,000 copies. So, we know they printed more 4e books initially than 3.5 and 3e books.
We also know that they sold out that first print-run a week before the game launched, which required a second print-run to be ordered. That second print-run sold out within two months after the game's release, prompting them to order a third print-run.
And while this technically means that 4E is so far a success, we don't know what the expectations were nor do we have any other evidence which hints at how good 4E really sells.
We can use precedent to determine their expectations might have been. 3e's first print run sold out in about 3-4 months (December 2000), prompting them to order their second print-run at that time. So, they might have expected a 3-4 month sell period to go through their first print-run, since that's what history showed them the market supported at one point. The fact that at the 4 month point in 4e's life cycle they were two complete print-runs ahead of that sales record speaks volumes about how well it's selling.
Pair that with the fact that the number one online direct-to-customer retailer in the world for books (Amazon) has them on their top 25 for 2008 (beating out a Stephen King novel, even), saying that we don't know if it's selling well is just denying reality.
We don't even know if WotC was talking about selling to stores or selling to customers.
We do know that WotC is a successful business that knows far more about the publishing industry than we do, so I don't see them jumping the gun and pumping out print-runs that aren't being sold through to customers, especially in light of the fact that they directly learned that was one of the things that killed TSR (overprinting unwanted product, resulting in a huge amount of stock with no value).