Go up to the Main Deck and take the Main Elevator up. Go through the door to the right that was broken open by the SA-X early on in the game. Don't go through the next door just yet; instead, make a running start from the right towards the left back to the Main Elevator room. You'll break through some blocks in the next room with a Speed Boost and discover a Missile Tank. Grab it and head right, follow the path through the Navigation Room (no need to uplink), and up the shaft beyond it. Take the green hatch on the left and take the elevator up to the Habitation Deck.
Use the door to the right to save if you want, then go back and enter the shaft to the left. Some free-floating X will tranform into a Fune and a Nahime. Make the Fune come out of the wall and freeze it, then stand on it and make the Nahime come out, and freeze it as well. Use it to get up to the door above, and go through.
You will now see the source of the bio-signs behind the glass: Etecoons and Dachoras, whom you met on Zebes. Pass the first block in the ground and find the second, and bomb it. You'll drop through the Pit Blocks below. Shoot the left wall to break it open. The block to the left can be broken with a Speed Boost, so run to the right, open the gate, and make a running start from the green hatch at the right, towards the left. Once you've fallen below, open the gate to the right and get the Missile Tank beyond.
Now head to the left. There is a passageway in the left wall that is hard to discern at first glance; jump into it to reach a secret shaft. Criss-cross up by taking the ladders, freezing the Fune when they come out, jumping on and continuing upward. At the top, exit right to return to the room with the habitat housing the Etecoons and Dachoras. Log into the console to free them. Will they be infected by the X now...?
Return to the elevator down to the Main Deck, where Samus will comment on the Etecoons and Dachoras.
Exit right, drop to the bottom of this shaft and uplink in the Navigation Room to the left. Adam confirms the deaths of all the station's crew members, whom the X used merely for knowledge, as the human form is too weak for battle. X have now appeared that are resistant to your current weapons. In anticipation, HQ has sent down Power Bomb data. You must download it in Sector 5 (ARC). Make your way to the Main Elevator and head down to Sector 5.