4X refers to a vieo game genre with four primary goals, namely to "eXplore, eXpand, eXploit, and eXterminate." The first 4X games were turn-based, but game developers have created real-time 4X games, as well. One of the best known examples of the 4X genre is Sid Meier's Civilization series, which has sold over 8 million copies.