Completing Area 4

Walkthrough: After going through the door at the very top of the previous section, jump across the platforms to reach the next lava room. Once you come to the wall on the far right of said room, you'll find another Super Missile Expansion. Get to the top of the Lava Room and run across the smaller section of the room. Bomb the breakable wall to find the next Gamma Metroid. The Metroid will constantly flee from one room to the next, so follow it the best you can and defeat it.

After doing so, exit the room by using the Super Jump to reach the ledge above, which will lead you back to the previous room. You'll have to backtrack from here, all the way back to the Save Station to be exact. From the Save Station, go left and enter the door with the green barrier around it. Head upward straight into the room, and then Space Jump all the way up to find the next Missile Expansion. By going to the end of the path, you'll find the room with the next Metroid, this time a Zeta Metroid.

After defeating the Zeta Metroid, exit the room the way you came and return to the Save Station. Open the next door with a green barrier at the bottom, then make your way back to the elevator which leads to the Chozo Seal. From the Chozo Seal, go all the way to the left and shoot a Super Missile at the block covering the red glowing orb. Use your Grapple Beam to pull and destroy it. From there, go over to the other side by passing the Teleportation Statue and going right. Go through the Morph Ball Maze, ending by using your Grapple Beam on the next red orb.

This should open up another path that leads you to the left. Follow it and go into the next room, moving upward to the open door that leads to the previous room. Use your Spider Ball to then roll through the passageway on the ceiling. This will lead you to the next Missile Expansion. After getting it, scale the ceiling again, but this time drop down on the breakable floor so you can drop into the next room. Once in the big room, defeat the final Metroid, an Alpha Metroid. Go back to the second Chozo Seal and drain the purple lava. Drop through the open area that is shown and head to the right. By following the path you'll come to more purple lava. Spin Jump across it to reach a door, the same that has the elevator that leads to Area 5.

Missile Expansion #1: Just before you make it to the Zeta Metroid, use your Space Jump to get to the top of the room. There should be a ledge in the top left corner that holds the Missile Expansion.

Missile Expansion #2: As you head to fight the final Metroid, use your Spider Ball toward the ceiling. Make your way across without dropping down, and you'll find the Expansion waiting for you.

Super Missile Expansion #1: Back in the first lava room, get to the wall on the right and look for the breakable spot. Try hanging over the ledge on the opposite side of the block and shooting a Super Missiles to reveal the Expansion. Get it quickly before the block reappears!

Super Missile Expansion #2: Before using the elevator to Area 5, use the Spider Ball to cling to the ceiling without touching the remaining purple lava. Grapple Beam the red glowing block you will come to, and then shoot a Super Missile to the left to reveal the Super Missile Expansion.