While the Research for "PS2 Emulaton on PS4" is still in progress showing that there are already a lot of old "PS2 Classics compatible through Emulation", it seems that the officials from Sony (Interactive Entertainment America LLC) have their own research on how to present Backwards Compatibility for the future, according to their latest patent application.
Not much is known at this moment due to the fact that there is no platform mentioned in this patent application. A Emulation of PS3 Titles running locally on the PS4 still sounds difficult - due to the complicated CELL B.E. processor used for the PS3 - and unrealistic since they would promote their PlayStation Now Service for that. It could be just a patent renewal for their actual Emulation process on PS3/PS4/VITA, so nothing special. But it could also be something completely new, even for a successor of the PS4 or even for Windows-Platforms etc. - no one knows.
Maybe the future will bring some more details. What do you think about the (secret) plans from Sony? Which console could be supported or which platform could be emulated?