The PS4 will use an IBM Power7 CPU according to Dutch site PS3Clan.nl. They go further saying that it was Sony’s decision to swap from the Cell BE architecture to IBM Power architecture that prompted IBM to cancel further development of the PowerXCell 8i CPU.
The Power7 is a CPU that has been under development at IBM since April 2006 and is not expected to be released as a product until mid-2010. IBM did reveal some of their design goals and intended specifications back at the wonderfully named Hot Chips conference back in August.
The headlines of those planned specifications are for the chip to initially be manufactured on a 45nm process and run at 4GHz. It will have 4, 6 or 8 cores per chip with each core will be capable of running 4 threads.
So according to the rumours that are floating around we now know both the CPU and GPU for the PS4. They are both plausible, but it will likely be some time before we know whether we can believe them.
Update: The original source for this rumour, like the GPU one, is FGNOnline.