Why should there be a 4th Edition, besides the presumed inevitability of it?
2nd Edition was a money-grab to print Players Handbooks that EGG wouldn't recieve royalties for, while purging all the things that the Religious Right objected to and introducing a few ideas from 1e suppliments that were working well (THAC0 and NWP's) and a few design ideas that didn't stand the test of time (only 4 core classes, with all other classes being a variant on the base 4).
3rd Edition was needed because the AD&D framework was showing it's age. Game design had progressed a long way, and the holes in 2e were legendary and gaping. Every group I knew played D&D as a heavily house ruled, heavily customized game that was only barely recognizable as D&D, and you had to re-learn it to play with a different group, because 1e/2e wasn't doing the job. 3e made D&D flexible, fast, easy, and consistent, and did it in style.
3.5 at least filled in a lot of gaps that 3.0 showed. The front-loaded ranger and Harm/Heal being famous ones, but it really wasn't halfway to a new edition, it wasn't anywhere near as huge as the 1e/2e or 2e/3e schism.
However, what huge gaping holes in design does 3.5 have? I don't like some 3.5 spells and their PokeMount, but that's certainly not enough to warrant a new Edition. 3.5 is overall the best designed, most flexible, easiest to play version of D&D I've ever seen, and I wouldn't trade it for any other game (although I would and do hybridize in rules from UA and AE liberally). If 4e comes out anytime in the forseeable future, I won't even pay attention, I've got the rules set I want and need right now. In fact, 4e would just mean that I could finally stop buying 3e books and complete my set and stop having to learn the rules and options from the new book-of-the-month.
As some of the gamers in my group have put it: "I don't like the idea of planned obsolescense in roleplaying games." I agree with that, any artificial target date for 4th edition, whether as a book-sales stunt to placate their Corporate Masters or just to fulfill the 10 year pattern risks being a failure. In time, enough new design ideas will trickle in, the gaming public will drift in it's desires and wants and styles of play, and between new ideas and new needs there may one day be a real need for a change big enough to warrant a new edition.
But that day is not today, and not in the forseeable future.