I'd guess a 75% probability there will be a PS4. If it does happen, it will be after a massive restructuring and change of strategy at Sony's gaming division.
The reason is simply that their strategy doesn't work anymore. The Playstation consoles have always relied on third party exclusives, even though Sony has a lot of good 1st party games. Microsoft came and stole that advantage by being richer than Sony, and this means Sony has to find other methods to bring in gamers.
First of all, say goodbye to the loss-leading model. Losing anything more than a few tens of dollars per console is not acceptable with the competition Sony has. They need breathing room to allow for unforeseeable low demand and high competition.
Second, they need to be more innovative. Differentiate themselves using gaming features, not media center ones. If they can't do this for the PS4, they might as well not launch a console as the Playstation brand won't save the PS4 as much as it saved the PS3.
Third, they need to stop acting as if they're still the leader of the console market. Many of their developers are too slow to release games, and that only works when those games aren't needed on a timely basis to prop up sales. The purpose of first party developers is to build up the userbase before 3rd parties are on board, and in this regard Sony's 1st parties have failed (I am thinking especially of GT5's absence here).
In any case, the biggest threat to the existence of a PS4 is the ongoing recession and resulting economic troubles at Sony. In a few years Sony may be a very different company due to this, so many of our assumptions may become irrelevant.
My Mario Kart Wii friend code: 2707-1866-0957
