As already said the ps4 simply isn't powerful enough to emulate ps3. The 360 was much simpler architecture and will still need patches to make each game run on xbone.
Personally I would be really happy with ps1 and ps2 compatibility. The ps4 is easy powerful enough to fully emulate ps2 and could do so with upscaled 1080p graphics no problem and other graphic filters to enhance them visually and wouldn't even require patches. There are many fantastic ps2 games that would be great to play again. I still have a collection of about 30 ps2 games, the favourites I just couldn't let go of. The ps2 gives absolutely terrible video quality with modern flat screen displays.
I honestly don't see why the ps4 doesn't have built in ps1 and ps2 compatibility.
