Think of it as an evolution of the PS3, rather than a whole new approach. Sony have been developing the PS3 constantly this generation and the PS4 will just allow their direction to continue into the future.

The big difference is very close developer discussions to create a system that is really easy to develop for, and in turn really easy to port game to and from. The PS2 and PS3 were both very hard to develop for as they had very complicate custom CPUs and GPUs. Looking at the PS4 spec sheet you will notice that the hardware is very much PC based for the PS4. This really reduced Sony's manufacturing costs for one, but also a x86 based 64bit CPU with 8 cores and a Radeon GPU mean any developers already able to code PC games can jump straight in and know what the hardware can do. 8GB of very fast bandwidth DDR5 ram (normally only seen on GPUs) opens the system up for much more than can currently be achieved. The biggest issue on current gen 360 and PS3 is the amount of system ram. When cross developing with PC current consoles have to take a bit to allow for less ram and slower bandwidth between parts.