stands for “Package.” It is the official installation file format used by the PlayStation 4 operating system (Orbis OS). When you download a game from the PlayStation Store, the PS4 downloads a .pkg file, installs it, and then launches the executable.
Because hosting copyrighted PKGs is legally gray, we won’t link directly. Instead, here are reliable sources and naming conventions to look for: God Of War 3 Ps4 Pkg