The "part4" designation indicates this is one piece of a multi-part compressed archive. To access the content, you generally need all parts (part1, part2, etc.) in the same folder.
Rare cosmetic outfits and instruments originally earned through online play. FUSER.ALL.DLC.part4.rar
These archives typically aim to complete the setlist, which originally featured: The "part4" designation indicates this is one piece
Are you trying to install these on the or Epic Games Store version? you generally need all parts (part1