Subchamber Four is a room in the Impact Crater. The room is accessed from the Subchamber Three, and leads to the Metroid Prime Lair through Subchamber Five.
Samus enters the room during the battle with the Metroid Prime. There are a number of orange roots high on the walls that serve no purpose. There are three grooves in the ground which Samus can use to avoid the Metroid Prime when it charges by rolling into the Morph Ball. Once the Metroid Prime has taken 100% damage, it will fall through Subchamber Five into the Metroid Prime Lair, and Samus will follow.
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