Lastly, WotC brand management and WotC R&D have a lot more control over their own products and their release dates than you might think. It's not really accurate to say that Hasbro even really thinks about D&D enough to force them to do a new edition--particularly if R&D doesn't think it's a good idea. Hasbro just looks at the bottom line. They don't care how the bottom line is upheld. (It's my understanding, for example, that 3.5 was a directive of the D&D business manager at the time, as well as an eager R&D staff, not a specific mandate from Hasbro.) So I'll argue that there are people at the switch who care about the game. There going to want to make 4e, whenever it comes out, something really special. If 3.5 showed us anything, it's that whenever 4e sees the light of day, it's going to most likely be the product of a lot of time and work. It's probably going to be fairly new and different. Probably at least a little groundbreaking. (I say that 3.5 proves my point because 3.5 could have easily been 4.0--it was as different from 3.0 as 2e was to 1e, more or less. So if that's a revision, when they set their sights on 4e it'll be something big, cool, and different. Whenever WotC does something like that, they do it with a lot of preamble and fanfare.)
I'll join the chorus of the people who say that it's premature to be worried about this, or even to give it serious thought. We'll know when 4e is coming. You won't have to hear it as a rumor or as pure speculation from some industry type at a con or on a messageboard.