I believe the 4E community is large enough that 5E can't succeed without it, and it doesn't need to be a majority for that to be true. A 4E rejection would compound 5E not being adopted by others, as I don't expect much of the OSR to adopt 5E and due to the fervor of a lot of Pathfinder fans there are going to be significant holdouts there.
While there's no way to be certain of the exact size of the old-school, pathfinder, and 4e factions of the fanbase (let alone how much crossover there is among them), it's not /that/ hard to see which of them WotC is courting, and we could speculate about why...
First, there's the official line: They're trying to bring everyone together under the 5e umbrella. That's probably mainly for Hasbro management consumption, it says: "all that 'lost revenue,' you see from Pathfinder and retro-clones, all those amazing sales from the 80s, we can get you that, honest! Let us try one more edition, please."
Then there's the playtest and all the L&L articles and what they actually deliver vs what they promise. The actual deliverables, are, as you might expect, a mixed bag of past edition bits. There's 'Hit Dice' which are like healing surges in way (but really not), and like hit dice in being, well, dice. But, the preponderance of it, and the whole 'modular' aproach points to a return to classic D&D, when variants apeared in the pages of The Dragon and every DM had his own house rules and the game was very fluid.
So, my conclusion, and this is mostly me reading between the lines and speculating, is that 5e is mostly going after re-capturing the classic D&Ders. They can't just re-print (though they're doing that, too) or make 5e into a retroclone, it has to be 'classic D&D, but more so, with the D&D nameplate' to win out against it's own past eds and retro-clones. The reason this makes business sense is that D&D was a fad in the 80s, and more middle-school boys played it back than then have played it in the two decades since. All those grown up boys, with 40-something incomes, are a prime target market.
3e and 4e fans are not really the market. 3e fans are getting all the fanservice they could ever want from Paizo, and nothing short of Heinsoo & Collins committing sepuku
or using a time-machine to prevent the publication of 4e is likely to get them back. 4e fans must, logically, contain some uncritical-early-adopters, fans of the D&D name who don't care about what's between the covers, who will just play anything and everything that's labeled D&D. WotC might have a pretty good idea of the proportion based on the sales of para-D&D boardgames like Dungeon Command and Castle Ravenloft. But, whether it's 10% or 90% or anything in between, they must figure the balance is expendable. Besides, it's not like there'll be a 4e-retro-clone competing with 5e. 4e fans will have the choice of playing the exact game they have now, with no future support, or giving in and playing 5e, or at least buying 5e to adapt new material to their 4e campaigns.
If nothing else, I can see the rancor of the edition war souring WotC on the idea of trying to service either the 3e or 4e splinter of the larger fan base. They might even figure that they'll get more of both, in aggregate, coming back if they do absolutely nothing for either, than they would get of one side if they just chose a side.