Metroid Fusion Sector 4 Walkthrough: Surviving AQA Without Getting Fried

You just got the Speed Booster from SRX, and now Adam is sending you into the drink. Welcome to Sector 4 (AQA). This place is a nightmare of electrified water and those annoying Scizors that block your path. Honestly, if you aren't careful, the environmental damage alone will end your run before you even see a boss. Most people struggle here because they try to rush through the water. Big mistake.

The Problem With Sector 4

Sector 4 is basically one giant puzzle about managing your health while navigating submerged corridors. Since Samus is still wearing the Fusion Suit without the Gravity Suit upgrade, your movement in water is sluggish. You're heavy. You jump like you're wearing lead boots. To make matters worse, the X-Parasites have sabotaged the electrical systems.

Touch the water while it’s electrified? You're dead.

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Well, not instantly, but your energy tank will drain faster than a leaky faucet. Your first goal isn't even fighting; it's finding the room to pump the water out or shut off the power.

Navigating the Initial Submerged Ruins

When you first drop into AQA, you’ll notice the water is shimmering with a nasty electrical current. Stay on the platforms. Seriously. Use your missiles to clear out the Oums (those shelled creatures) from a distance. You'll need to head to the right, navigating through several rooms where the floor is almost entirely submerged.

If you see a wire hanging into the water, that’s your red flag.

You’ll eventually reach a room with a save point. Use it. Metroid Fusion is notorious for its sudden difficulty spikes, and the path to the Diffusion Missile is one of them. From the save room, move deeper into the sector until you find the pump control room. You can't actually fix the electricity yet, but you can lower the water levels in specific areas to make progress possible.

Hunting the Serris X

This is where the Metroid Fusion Sector 4 walkthrough gets intense. You’re looking for Serris. Serris is a high-speed sea serpent that has been infected by the X, and it’s arguably one of the fastest bosses in the entire franchise.

Before you get to the boss room, you’ll find yourself in a large vertical chamber. You need to use your new Speed Booster. Run across the top platforms to break through the Speed Booster blocks on the right side. If you mess up the timing, you’ll fall into the water and have to climb back up while taking chip damage from the electrified surface. It’s frustrating. It’s tedious. But it’s the only way forward.

The Serris Boss Fight Strategy

Serris doesn't play fair. It moves in a predictable "sine wave" pattern, but it moves so fast that tracking it with your thumbstick or D-pad feels like a chore.

1. Stay on the platforms. Do not fall into the water. Even though Serris is fast, the water is still electrified during this fight.
2. Aim for the head. That’s the only weak point. Missiles are your best friend here.
3. The Invincibility Phase. After you hit Serris, it will turn red and move even faster. It becomes invincible during this time. Don't waste your missiles. Just jump. Time your jumps so you’re at the peak of your arc when Serris passes under you.
4. Charge Beam vs. Missiles. While missiles do more damage, a well-timed Charge Beam shot can sometimes be easier to land if you’re struggling with the serpent’s speed.

Once Serris is toast, you get the Speed Booster... wait, no. You already have that. You get the Speed Booster upgrade's full potential by absorbing the Core-X, which allows you to move at supersonic speeds and perform Shinesparks.

The Long Trek to the Diffusion Missile

After Serris, Adam gives you a new objective. You need the Diffusion Missile. This is the real prize of Sector 4. The path leads you through a series of "Level 4" security doors (the red ones). If you haven't hit the security terminal yet, you'll be doing a lot of backtracking.

In the upper-left quadrant of the map, there’s a hidden path. Most players miss it because the ceiling looks solid. It’s not. Use a bomb or just jump and shoot upward. You’ll find a crawlspace that leads to the Data Room.

Why Diffusion Missiles Change Everything

Once you download the Diffusion Missile data, Samus becomes a powerhouse. By charging your missile, you can create a massive ice explosion that freezes everything on screen. This isn't just for combat. It’s a navigation tool.

You’ll use these to freeze the "Ripper" type enemies to use them as platforms. In Sector 4, this is the only way to reach some of the higher Missile Tanks and Energy Tanks hidden in the rafters. There’s one specific Missile Tank hidden behind a wall of vines that requires you to shoot a Diffusion Missile at a nearby wall—the blast radius is the only thing that can reach the trigger.

The Return Trip and the Golden Scizor

Getting out of Sector 4 is harder than getting in. The X-Parasites are onto you. As you head back toward the elevator, you'll encounter a Golden Scizor. Normal shots don't work. Missiles don't work.

You have to hit it from behind or use a fully charged Diffusion Missile to stun it. Honestly? Just jump over it if you can. It’s a resource drain that you don’t need right now.

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Keep an eye on the background. You’ll see shadows moving in the water tanks. The game is constantly hinting that the SA-X is nearby, but in Sector 4, your primary enemy is the environment itself. Keep your health above 200 at all times. One mistake in the electrified pits will send you back to the last save station, and losing 20 minutes of progress because of a bad jump is a rite of passage for Metroid fans.

Essential Tips for Sector 4 Mastery

• Look for cracks. The walls in AQA are fragile. If you're stuck, power bomb or regular bomb every corner.
• Shinespark Secrets. There are at least two Missile Tanks in this sector that require a "hidden" Shinespark. You have to start your run in the long hallway leading to the Serris room, store the charge, and blast upward in the adjacent vertical shaft.
• Watch the bubbles. In the submerged areas, bubbles often indicate where a hidden passage or a vent is located.
• Don't ignore the fish. Even the small Skultera fish can drop large X-Parasites. If you're low on health, farm them before moving into the next major room.

Moving Forward

Once you've cleared the main objectives in Sector 4, your next stop is typically Sector 3 or 5, depending on how much Adam wants to boss you around. The Diffusion Missile you just picked up is the key to unlocking the rest of the station.

Actionable Next Steps:

1. Check your Map Completion: Before leaving Sector 4, ensure you’ve found the hidden Room in the bottom right; it contains a Power Bomb tank that you'll need for the late-game.
2. Practice the Shinespark: The hallway before the elevator is the perfect place to practice "storing" your Shinespark charge (crouching while glowing). You'll need this skill mastered for the final sectors.
3. Save at the Navigation Room: Always touch the Nav Map before heading to the elevator to ensure your progress is logged and you get your next briefing.