In Halo: Combat Evolved, most of the game takes place on Installation 04.
Installation 04 is one of the enormous ring-shaped installations built by the Forerunners. It is also called Alpha Halo and is commonly known as the first Halo ring encountered by the player. The human ship Pillar of Autumn arrives there after fleeing the Covenant attack on Reach.
The ring initially appears to be an ancient weapon and a possible strategic advantage. As Master Chief and Cortana investigate, they learn that its purpose is connected to the Flood and that activating it would have catastrophic consequences for sentient life in the galaxy.
Installation 04 is destroyed near the end of the game when Master Chief and Cortana trigger the unstable Pillar of Autumn’s engines. It should not be confused with Installation 05, the setting of much of Halo 2, or Installation 00, the Ark.