Installation 04B was a partially completed Halo installation which was under construction at the Ark since the destruction of the original Installation 04. The Installation was designed as a very similar replacement to the original Installation 04 in order to ensure that the Forerunner's Halo Network was intact, and therefore able to encompass the entire Milky Way galaxy.
Following the Battle of Installation 04 and its subsequent destruction, Installation 00 was notified, and an automated function began building a new replacement for the missing Installation's position. The main weapon was built first with the surrounding circular metal structure after it using materials that came from the Ark's Core, a planetoid located at the center of Installation 00.
Before it was released into space, the Installation had a number of primary features available, even though it was still not complete. These included the environment, atmosphere and main structures, and the main weapon, though the ring structure was not yet stable enough to survive this weapon's discharge. The new Installation was also not networked to the other Installations, allowing a tactical pulse to be fired without triggering the other six Installations. It was never revealed if this is an option available to completed Installations as well, although it is likely that Installation 04B merely had not been set up with this network yet, as it was incomplete; a logical precaution considering the effects of a misfire.
The new Installation was released into space just above the Ark, but it was still incomplete at that moment. John-117 and Thel 'Vadam decided that the only way to stop the local Flood infestation (which included the Gravemind from Installation 05) was to fire the incomplete Installation prematurely because it was out of the range of the other rings.
John-117 headed to the Flood-controlled High Charity to retrieve Cortana, as she held the Activation Index from the first Installation 04. Aided in his escape by Thel, John found a damaged Pelican near the exterior of High Charity and flew it to the Control Room. The Pelican crash landed in a snowy canyon near the Control tower. As John and Thel made their way towards the tower, Flood dispersal pods descended from the sky to prevent the two from activating the new Halo. The two, aided by Sergeant Major Avery Johnson, reunited with 343 Guilty Spark. When Sgt. Major Johnson tried to force-fire the new Installation, the Forerunner Monitor interrupted the process because of protocols and possible hints of rampancy, and engaged the three, mortally wounding Johnson. With 343 Guilty Spark eliminated, John-117 activated the new Halo with the Index kept by Cortana.
- Main article: Raid on Installation 04B
As Spark predicted, the new Installation was not ready to fire, and it began to shake itself apart and disintegrate. John-117 and Thel 'Vadam escaped through the second Phase Pulse Generator chamber and proceed towards the Dawn using Sgt. Major Johnson's Warthog. The shockwave of the activation began to tear the Installation apart, causing explosive reactions all across incomplete sections of the Installation. John-117 and Thel 'Vadam barely managed to reach the Dawn before the Installation exploded.
- The multiplayer map Avalanche is located on Installation 04B, but the Installation 00 and the Milky Way galaxy are missing from the skybox.
- Like all Halo Arrays, Installation 04B get its power from the Power Rooms which are connected to other Power Rooms placed around the whole ring. A power room on Installation 04 featured a large vertical Core and a dome-shaped roof, with large glass windows that run down the side. The energy which powers the Installation can be seen pulsating up through the core and then back down through the windows.