The specific you are seeing (e.g., "Invalid CD Key", "Registry missing")

If you are moving a retail installation, the game might ask for your CD key when launched on a new computer. Open your q4base folder. Look for a file named quake4.key or q4key .

The phrase "CD key" is the second pillar of this search query. Quake 4 uses a 5x5 alphanumeric key (e.g., Q4-XXXX-XXXX-XXXX-XXXX). Without it, you cannot install the game or play online.