While Sony effectively phased out hardware emulation of previous platforms during the early stages of the PS3 era, it has worked consistently on opening up the PlayStation back catalogue to publishers via software emulation and PSN downloads. Internally, PlayStation 3's firmware contains emulators for PS1, PS2 and even the handheld PSP, while Vita is capable of running PS1 and Vita titles virtually flawlessly using the same technology. In short, there's a proven codebase that can only get better for the vastly more powerful PS4.
Our information suggests that the same internal emulator strategy is planned for PlayStation 4, and we understand that Sony is actively pursuing the ability for older titles to run without the blurry upscaling seen on PS3, suggesting that native HD resolutions are being targeted. Assuming this intention carries through to final code, we'll be seeing an effect similar to the resolution scaling seen on unofficial PC emulation of Sony's consoles, as well as a great many of the "HD remasters" we saw on PS3 - where original assets were rendered at a higher resolution, often without any actual remastering at all.