Line Installation 1-4 served as a Flood Research Facility, as well as a defensive installation, as it possessed a powerful energy weapon system that was capable of shooting down any ships passing nearby, even when in Slipspace, causing them to crash down on the moon. The Installation had its own Monitor, named 686 Ebullient Prism, which had a sizeable force of Sentinel and special Gatherer units under its command. The Installation itself is a vast underground complex with tunnels and hallways reaching kilometers deep. Most of the tunnels appear as if they had been designed with flying machines such as Sentinels in mind, as vertical or angled service tunnels are more frequent than walkways.
Due to the Installation's active defense system, the moon's surface became littered with wreckages of ships over the course of history. During the Human-Covenant war in the mid-2500s, the Long Time Coming, as well as the Covenant Assault Carrier Clarity of Faith were shot down by the Line's defenses while in Slipspace. As a result, they both crash-landed on the moon and some of their personnel were taken by Gatherers into the main facility of the Installation, for study by 686 Ebullient Prism. During the ensuing conflict, the installation's control center was located and used to shoot down an entire Covenant fleet in Slipspace.
- The Installation is apparently a defensive structure used to shoot down infected ships during the Forerunner-Flood War, and may have been linked to the Maginot line. This is supported by the Installation's name, as well as its function. In addition, Frank O'Connor commented on the name that "fans of the terminals will probably figure it out", implying that the reference to a "Line" is linked to the story told in the Terminals of Halo 3, where the Maginot Line was first mentioned.
- The Installation's control center appears near-identical to that of the Relic in Halo Wars, complete with holograms of star systems.
- An unidentified Line Installation was featured in the UNSC Infinity's War Games simulation, as the setting of the multiplayer map Wreckage. It is unknown if it was Line Installation 1-4 or a different one. Halo 4: The Essential Visual Guide later confirmed it was actually a different one, Line Installation 9-12.
- Halo: Blood Line (First appearance)