Since the PS3 was launched the Cell has gone from being manufactured using a 90nm process to a 45nm process which means that they could (theoritically speaking) fit 4 cell processors on the same die as they initially produced the cell processor on; by the time the PS4 comes out (in 2011/2012) it is possible that it will be manufactured using a 22nm process which means you would be able to fir 16 cell processors on the same die as they initially produced the PS3's cell processor on.
The processor that is used in the PS4 will probably accept the same instruction set as the Cell processor from the PS3 but I suspect that its architecture will have changed quite a bit in order to make it easier to develop for, and to increase its real world performance.
