As reported by emulation fan Read Only Memo, the team behind the RPCS3 PS3 emulator (which started life back in 2011) seems to be working on a PS4 emulator called RPCSX - as spotted on Github. As pointed out in the original blog post, the project appears to still be in the early stages of development, and isn't the first of its kind, with plenty of other developers also trying to create their own PS4 emulators of varying quality.
Speaking to one of the project's developers, Read Only Memo discovers that the idea for a PS4 emulator has been floating around amongst the team for a while. "I did some things just after the console's release," the developer says, however, they decided to wait until after the release of the PS5 to work on it further.