Would you believe that the PlayStation 4 remaster of Parappa the Rapper is actually the PSP version running under emulation? That's the remarkable claim put forward by hackers using compromised PlayStation 4s to examine game code, first revealed by 'KiiWii' on the GBATemp forum (with a tip of the hat to Ars Technica for its write-up of the story so far). The actual remastering - such as it is - appears to come in the form of a high resolution texture pack that swaps out the original PSP assets for renditions more becoming of a current-gen system.
The evidence here is compelling, with several hackers providing proof of a small number of other PSP titles running on PlayStation 4 using the same emulation wrapper. Essentially, within the PS4 package, a configuration file points to a PSP UMD disc image, which - in the case of Parappa the Rapper - apparently has the same MD5 hash as the original PSP game, confirming that it's original, untouched code. The configuration file itself suggests that Parappa is running under an all-purpose emulator, with a number of options including the ability to choose between multi-sampling and super-sampling anti-aliasing, plus a toggle on the L3 button to swap between original textures and any remastered replacements.
Further research from hackers has led to a small number of other PSP games running on PS4, apparently confirming that it is a general emulator working behind the scenes here, as opposed to any code bespoke to Parappa only. Compatibility does seem to be highly limited at the moment though. The implication is that further PSP titles released on PS4, such as Loco Roco and Patapon, may also be using the same technique, as injecting the original PSP code for those games into the emulator seems to work fine. Since the original Parappa revelation, 'injector' tools have been released that swap out the Parappa UMD image for one of the user's choosing, with the resultant files only able to run on compromised PlayStation 4 consoles. The list of games running on the emulator seems to be very limited, however.