Files with the associated extension .PK4 are compressed archive files used by the game 'Doom 3'.

These files are also known as 'pack' or 'pak' files, and are essentially the same as normal 'Zip' files, i.e compressed data files.

Compressed files reduce the file size, which is beneficial for data storage or for data transfer over the internet, etc.

With regards to Doom 3 the compressed files extend loading time, but severely reduce storage requirements, with the uncompressed Doom 3 file being 3.6 GB in size, while the compressed version being only 1.44GB.

Doom 3 is a first-person science-fiction style shoot-em up.

.PK4 files can be uncompressed using 'WINRAR', 'WINZIP' or '7-zip'.