Backtracking After Area 4

Walkthrough: After finishing Area 4 and earning a number of new weapons and abilities, you can actually backtrack in Area 1, Area 2, and Area 3 to obtain various collectibles in hopes of reaching closer to the 100% Completion mark for those given areas.

Missile Expansion (Area 1): In the top section of Area 1 is another door with a purple creature on it. Use your Spazer Beam to blast away the monster and earn access to the door. Once inside, use your Spider Ball to scale across the ceiling to reach the Missile Expansion.

Missile Expansion (Area 2): In the top right corner of the map is another Missile Expansion sitting on a ledge. You couldn't reach it before, but now that you have the Space Jump, you can easily obtain it now.

Missile Expansion (Area 3): Head to the big and open room at the top of the area. From there, use the Space Jump to reach the ledge in the top left corner of the room to obtain the Missile Expansion.

Super Missile Expansion (Area 2): In the top left corner of the map is a door you passed earlier with a green barrier. Use your new Super Missiles to gain access to the door, and once inside, Bomb the two set of weak blocks just to the left. On the third row of said blocks, make sure to Bomb the hidden block as well. As you as you drop the Bomb, begin a Space Jump so you can grab onto the ledge and obtain the Super Missile Expansion.

Aeion Tank (Area 3): In the rightmost of the vertical rooms toward the lower section of Area 3 is a small side room that is blocked by another of those doors with purple creatures attached to it. Defeat the creature with a Spazer Beam, and the Aeion Tank lies just inside the door.