I think there will be a PS4, but the better question is when, and how much? A lot of this depends on the PS3 itself. If it continues to struggle throughout its lifetime, then the PS4 will need some early success to regain third party support(similiar to what Nintendo is going through with the Wii now). If it starts picking up in sales down the line to become the market leader or at least hold a significant amount of the market share, then it'll show the PS4 can have a slow start and still turn out successful well.
The PS3 may or may not last 10 years, but it's very doubtful that if Sony's gonna stay in the console business that they'll skip a generation. This means the PS4 will likely launch in 5 or 6 years. I would think Sony would be making a profit on a sub $200 PS3 by then and the difference in price between the two consoles will likely be around $200. If the PS4 uses Cell, Blu-Ray, a GPU based off the RSX(which would help with BC if they decide to push it again) then Sony may actually make a profit off each console sold at launch for once.
I think a $300-$400 launch in 2011-2013 is likely.
