Unlike modern Steam or Epic keys, a Quake 4 CD-key was a 5x5 format code (e.g., XXXXX-XXXXX-XXXXX-XXXXX-XXXXX ). It served three primary functions:

Steam-Issued Activation Keys - Steam Support

When purchasing from third-party sites, reviews highlight different levels of risk: :

: The CD key is typically found on a printed sticker inside the game’s packaging, often on the back of the manual or the disc tray.

: These are 20-digit codes found inside original 2005 physical DVD cases. Warning: These keys often do not work for Steam activation. Many users have reported "invalid key" errors when trying to use old retail codes, sometimes because the original owner still has the key active online. Merchant & Marketplace Reviews