Paul Holman (Sony's VP of Technology) claims that "a PS4 will be launched by Sony but not until at least 2010." Reason being, "Sony will introduce a series of firmware upgrades that will give the PS3 more media centre capability while allowing for the introduction of third party applications and hardware 'such as interactive controllers' similar to the Nintendo Wii he said."
"The PS3 has an awful lot of processing power and we will harness this power with firmware upgrades and new features. He also admitted that the PS3 will become as much a media centre for the home as it is a gaming machine and the introduction of new capabilities such as the ability to download third party operating systems to the PS3 will see several new capabilities added to the system. We have already seen one European Linux application running on the PS3 and over the next few months we know of several others that plan to launch a product for the PS3"[via Smarthouse]