You don't even need wikipedia, just the four words that named the genre. "Explore, expand, exploit, exterminate."
Civilization, Galactic Civilizations, SoaSE, Aurora, Master of Orion, Sword of the Stars, and so forth. The only real divisions tend to be with grand strategy (i.e. every game ever made by Paradox) and RTS (Starcraft, C&C, etc.). 4X games are longer-term than RTS (and can be either turn-based or real-time) and tend to have a different focus than grand strategy (mainly in that grand strategy games tend to start with predefined historical scenarios as opposed to tabula rasa and proceed through a complex evolution of minor diplomatic and political changes rather than discovering new star systems or fighting enemy fleets).