I think its quite clear looking at the Vita, that the PS4, will be a very much off the shelf component design, its cheap, its low risk, developers love it, the only real question is will it be powerful enough to emulate the cell, I personally doubt it and thus no backwards compatibility again, but at least we will get PS2 back for sure, and lets face it thats the best generation to have as BC.
But im sure they will be working hard to get PS3 emulation, not so much for box products as those can be sold, but because people have built up a library of downloadable titles, and they will be pissed if they are unplayable and dead money.