"Pack.part04.rar" is a ghost of a larger file. By itself, it is a locked box with no key. If you found this on your hard drive, it's likely a leftover from a failed or forgotten download. Unless you have the accompanying parts and a trusted source for what was inside, it is generally considered safe to delete.

Downloading isolated .rar parts from the internet carries significant risks:

You usually only "extract" the first part, and the software automatically pulls data from the subsequent parts (like part 04) to rebuild the original file. The Mystery of the Name "Pack"

Do you have the of this archive, or are you trying to recover the contents of a corrupted set?