I think a 4.0 is certainly possible, and sooner than most folks think, perhaps in a couple of years. The designers of 3E wanted to do more sweeping changes to the game than they did, but were afraid of a backlash from the players. They've certainly got more ideas than ever now on how to improve it. And as mentioned, there are financial reasons to do so.
A significant update to the system would be a good thing, and not just financially. I would rather buy new core books every four or five years than see the game stagnate (again). No matter how good a system is, it should change over time to take advantage of better ideas. Change for the sake of change (or money) is a bad idea, but I'm sure the game can be improved after every four or five years of play.
If nothing else, a more complete core system, with basic rules for mass combat, guidelines for creating new monsters, feats, and prestige classes (stuff currently in Dragon), and the basics of books like the Epic level handbook and Deities and demigods, would be welcome.