Funnily enough, the Head of SCE was asked the very question ‘when can we expect the PS4′?
He said that they havent even fully launched the PS3 yet and it was far too early to be talking about a PS4.
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Since then there has been more talk that they fully expect it to last well into the next decade due to easy upgrades via software downloads.
Saying that, he didnt deny it outright. Although we may be talking about 6 years or so in the future  it is possible that SONY engineers have, at least, formulated ideas as to what the next Playstation will be capable of.
I personally think they havent got anything down in writing. I believe that they will see what new technology presents itself before even starting development on the PS4 (or whatever it will be called.)
Dont forget George Lucas didnt even consider making Star Wars 1, 2 and 3 until the new CGI technology was around to make it possible.
I reckon SONY will wait for a technology jump.
I dont think they need to change anything about the PS3 to make it last several years. As it PC based they can easily bring out Cell, perfomance and storage upgrades.
I also reckon that the next ‘console’ will be 100% online based. Sure, you’ll be able to download games onto a physical hard drive but why bother? Internet connections, bandwidth and speed could be so good in the next 6 years ….
If we dont need storage and a place to put disks in then we dont really need a console. It could be just a small box that sits on an internet connectioin point, or sit inside a TV.
The only thing you’d need is a controller !
…or maybe the controller could actually house everything you need! 
Exciting stuff….
I expect my grandchildren will be playing hologames, like 3D holographic chess in Star Wars..
 :) 
