PKG is an archive format used to store digital versions of games on the Playstation Network. The file contents of a PKG are encrypted, but the decryption keys have been found.
In fall of 2017 PSVita developer The Radziu released a pkg decrypter for PSVita pkg on homebrewed systems. later in winter that same year Ps4 developer RedEyeX32 released a tool to allow decrypting ps4 pkgs on homebrewed systems.
Several Persona games have been released on the Playstation 4, such as Persona 4: Dancing All Night, Persona 4 Golden, Persona 4 Arena and Persona 4 Arena Ultimax, and Persona 5. This means if you want to mod them you must have access to homebrew on the PS4. Homebrew currently is restricted (at least publicly) to system firmware 5.05 (or 5.07 which is basicly the same) and under which was released January 18 2018. Firmware 5.50 was then released March 8th and Sony requires games to be on the firmware that is most recent to when it shipped out to consumers. Its impossible to pinpoint exactly the time between a games initial batch of copies being printed and when devs get access to the then most recent firmware but a couple of weeks is probally a safe bet. Any games released after that point such as Persona 3 and 5's dancing spin offs ps4 versions and Persona 5 Royal require a higher firmware and thus as of writing are not hackable.
PKGView can extract all the files in a decrypted PKG. Just open the downloaded PKG file, right click the topmost directory and extract to source folder.