Wait, so...Why DID 4E underperform? Or is that outside your expertise here? (No shame if so, you're a game designer, not a market analyst. You can tell by the having a soul).
(With reference to this post here.)
There were a couple of major factors in play there.
First, a big chunk of Dungeons & Dragons 3rd Edition's popularity had come about due to robust third-party support, published under the auspices of Open Game License (OGL). Dungeons & Dragons 4th Edition, however, was not licensed under the OGL, being subject to a much more restrictive license imaginatively titled the Game System License, or GSL. Hasbro reportedly refused to negotiate with existing third-party publishers to get them on board with the GSL, or to offer transition support of any kind; instead, they simply demanded an immediate halt to the publication of all 3E material, and attempted to bludgeon publishers into compliance by threatening to yank their trademark authorisations (i.e., the agreements which allowed them to put the "D&D compatible" logo on the covers of their books).
Predictably, this approach was not well received. The largest of 3E's third-party supporters, Paizo Publishing (now Paizo Inc.), elected to produce their own game which was statblock-compatible with 3E in order to provide a venue for other publishers to continue producing OGL material; many of their peers decided to gamble on Paizo's plan rather than play ball with Hasbro, and this is how we got Pathfinder. Hasbro's behaviour thus caused D&D's third-party support to crater nearly to zero with the publication of 4E and created D&D's largest competitor.
Second, 4E was badly behind the curve on digital availability. Shortly before 4E was scheduled to drop, the digital masters (i.e., the files provided to printers in order to manufacture the books) were leaked on file sharing networks. Hasbro responded by panicking and ordering an immediate and indefinite halt to e-book publication of D&D material (in spite of the fact that the leak had demonstrably originated from their print production arm rather than their e-publishing arm), even going so far as to refuse to honour pre-orders for 4E's now cancelled e-book version.
Combined with a series of mismanagement-induced delays which caused 4E's virtual tabletop tools to miss the game's publication date entirely, and a decision to paywall what few first-party resources did manage to hit their target behind a monthly subscription, this resulted in 4E being available exclusively in print for the first two full years of its lifespan, at a time when D&D's competitors – including the aforementioned Pathfinder – were literally giving their core rules away in digital form for free.
As you say, I'm no market analyst, but I have a strong suspicion that "alienating practically all third-party publishers for a game line which was critically dependent on robust third-party support", "being the first edition of the game ever to face significant direct competition", and "making a game which dropped in the middle of the worst economic recession in thirty years available exclusively as an expensive printed set" resulted in the 4E stepping up to the plate with three strikes already against it. Add to that the almost comical ineptitude of Hasbro's advertising for 4E, and the usual drama of any major edition turnover, and... well.
