Speaking to T3, a Sony rep made a statement that discourages the thought of expecting a PlayStation 4 anytime soon.
"PS2 was still going strong after eight years, and with the power the Cell processor provides, not to mention the fact that PS3 is inherently future-proof, we see the PS3 sales curve far outlasting that of PS2."
http://gameindustryinsider.blogspot.com/2009/02/apparently-ps3-is-future-proof-sony.html