While Sony is rather strict with the hacking of PS4, it is possible to exploit the hardware on a lower firmware and gain access to it, which means you can technically install homebrew on it or run other unsigned code. This has led to a new discovery by some hackers who managed to gain access to the source files for PaRappa remaster for the PS4 and found out that it actually contained a full functioning emulator for the PSP that was programmed to run on the PS4.
As repoted by ArsTechnica, the emulator was originally discovered by GBATemp user KiiWii who shared this in a thread. It was a pretty basic but functional emulator and the remaster was made with a texture update that was created for 4K and redrawn artwork. The base code was still the PSP version so it was not a completely reconstructed game on the PS4.
This has resulted in some other hackers joining in the emulation of more PSP games being run on this emulator including Patapon and Namco Museum. It is still not possible to play every PSP game though, since the emulator has some internal limitation set in place including with file size and other checks that have to be figured out first before it can be modded into something else. Regardless this is an interesting discovery and makes the case for Sony using emulators but limiting games that can be allowed on it, even going far to not allow any backwards compatibility for their console unlike Microsoft.