I'd vote for shorter, myself. The business model is based on 'crunch' books, and the more those proliferate, the clumsier any system gets. I feel that 4e is probably going to start out quite streamlined -- so did 3e, really -- but the whole idea of a new PHB annually is going to make it get clunky really fast. There will be a point where sales of additional PHBs will drop off, because people won't want to spend more money just to add more complexity and bureaucratic rules headaches to the game.
At that point, they'll issue 5th edition -- unless the dying U.S. economy has continued its arc towards ruin, of course, and sales are so low that WotC goes out of business.
Openly embracing the "crunch explosion" publishing model is going to make 4e topheavy and clumsy even faster -- so 5e will come sooner. I have no doubt that 4e will start out as a truly beautiful system, but it's doomed to inflate quickly into a hopeless mess just through the very nature of the market.