Sector 4 (AQA) is the aquatic area of the Biologic Space Laboratories research station.
Sector 4 is made up of an upper area and a lower area. The upper area has large amounts of advanced technology and is where Samus faces Serris. The area also contains many tanks filled with water that are electrified due to power nodes broken by Serris. The computer informs Samus that the power cannot be cut because it would shut down the entire station and greatly hinder her attempts to combat the X. Instead, Samus must lower the water level using the Pump Control Unit.
The lower area is completely underwater, so the Gravity Suit must be acquired before Samus can explore it fully. This area has very little technology compared to the upper area and resembles a coral reef. The terrain in the lower part of the sector is very similar to that of Earth's oceans. This area is where Samus unlocks the Level 4 security doors. Samus cannot reach this part of Sector 4 from the upper area at first and therefore arrives by going through Sector 5.
"Sector 4 is used to research marine creatures."
"Sector 4 has been constructed to support a delicately balanced water habitat."
The theme heard when uplinking with Adam after defeating Serris is Facing a Huge Reaction.
- Sector 4 is the only sector in Metroid Fusion where the entrance is placed to the right, rather than the left.
- In the first large room of Sector 4, fresh blood can be seen dripping out of wall vents in the background.