PK4 files are a type of archive file used by Doom 3: Resurrection of Evil to store game data, including levels, models, textures, and sounds. These files are essentially ZIP archives with a .pk4 extension, which can be easily created and edited using standard archive software. The use of PK4 files allows modders to create and distribute custom content, such as new levels, characters, and game modes, without requiring extensive programming knowledge.