.fpk files can be found in the installation location of «Sid Meier's Pirates!» and contain most assets of the game.
.fpk is just a zip of files. It's probably just a format they quickly invented to prevent people from messing with their assets. It's pretty simple, but the filenames are 'obfuscated' by adding 1 to each letter.
Apparently, Firaxis (game devs) released a «Civ4 PakBuilder» tool to pack/unpack
.fpk files: https://forums.civfanatics.com/threads/civ4-pakbuild.136023/