There has been a lot of criticism of the Cell CPU. Granted there are gaffs in the design that has made it more difficult to program for and to fully utilize but to assume that because of the flaws of the original Cell CPU, they won't use an updated Cell is unreal.
What's not mentioned is that they will have had at least five years to update the Cell and we would have to expect that they will have fixed some of the shortcomings of Cell in a successor.
And as BenSkywalker points out, the development tools are much more mature now than when the Cell was first introduced. I don't see a bumpy transition in continuing the use of some form of the Cell processor in a Playstation 4.
In fact, it'd make a lot of sense considering the financial investment that Sony has made into the Cell processor. If they were going to switch to a "regular" Power based CPU or x86 CPU then they should never have invested in Cell in the first place.