I don't think the success or failure will be a question of when 5th edition will be out, but more one of whether or not WotC will cancel or sell the D&D line if it proves to be unsuccessful by their standards. If they are forced to invest a million or two to reinvent the game and then they only barely make that back, or they don't even get out of it what they put into it, then Hasbro will say that RPGs are no longer viable and they'll end their relationship with it. They may decide to license it out, which would probably be the best situation if a failure occurs, or they might just decide to put it on ice until they can come up with a newer, better strategy for making it appeal to the masses. At that point, the next edition might be radically different than any version of the game we've seen so far, and it might just be developed by the folks in Rhode Island rather than the ones in Seattle.