The CD key for "Sim City 4" is a code to unlock legally purchased copies of the game. The codes have twenty characters broken up into five sections of four characters each.
The CD key protects against piracy and illegally downloaded copies of "Sim City 4." However, some legitimate copies of the game come with unusable CD keys. If this is the case, exchange the game for a new copy or contact the manufacturer for a different CD key code. If you have the receipt and the original game box, it is much easier to receive a new code than if you have no documentation.