Sector 4 (AQA) was the aquatic area of the Biologic Space Laboratories research station. It housed various X Parasite-infected creatures such as Serris, Sciser, Skultera, Owtch, Gold Sciser, Powamp, Evir, Space Pirates and Yard.
Sector 4 was made up of two areas: 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 tells Samus that the power should not 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. 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 the only sector in Metroid Fusion where Samus must go left to enter the sector. All the other sectors have her go to the right.
- In the first large room explored of Sector 4, fresh blood can be seen dripping out of wall vents in the background.
- The theme heard when uplinking with Adam after defeating Serris is "Facing a Huge Reaction", named so in the tracklist for the Metroid Prime & Fusion Original Soundtracks. The theme of upper Sector 4 is titled "Sector 4 (AQA) Aquatic Control Level Area", and the theme of lower Sector 4 is titled "Sector 4 (AQA) Underwater Area".
"Sector 4 is used to research marine creatures."
"Sector 4 has been constructed to support a delicately balanced water habitat."