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If you've already read this intro for a different ending, please skip to #1.
A few words about Sector 4. We will begin at the area labeled Page on the UPPER LEVEL. All these end maps (for Helios, Dark Ages and Illuminati) are the same except for the number placement; in other words, all word labels remain consistent. The Sector 4 map may seem confusing at first, but really makes sense as you explore the level and is very accurate. North is up.
There are three levels to the area, the UPPER LEVEL map on top, MID LEVEL map at lower left and LOWER LEVEL map on the right, which I've labeled on the map. You can basically stack each of these levels on top of another, with the central area on each being where Page is.
There are a total of three Universal Constructors on this map, one on every level, labeled in each case: UC-03, UC-02, and UC-01. Each one of these respawns any enemies you kill. On this UPPER LEVEL, there are a pair of spiders roaming the north corridor near the UC. On the MID LEVEL there are a few gray aliens. And on the LOWER LEVEL, there are several karkians and greasels way below. There are no more troops in the game, unless you are backtracking to Sector 3 or choose the New Dark Ages ending. When you're on any level, you are safe from the creatures on any other level.
Fortunately there's a great way to stop their respawning and defeat them as well. On the MID LEVEL and LOWER LEVEL you'll notice I put the labels "Bot". At this point on each level is a security computer terminal that you can hack to release a nearby security bot, which then comes out of its cubby hole and destroys all the beings on the level. It will continue to do so until it's destroyed, protecting you. After you release the bot, it's a great time to go over to the UC door, blow it open with a LAM or use your multitools on the keypad, enter the small room and turn the giant dial, which traps all the spawning creatures on the level, making it safe for you to roam around at your leisure. I strongly recommend doing this on both MID and LOWER levels. The spiders on the upper level will only have to be passed at most once, so no need to worry about the UC up there. Now on with the ending walkthrough.
From Page, take the south corridor out of the room to #1, remembering to hug the walls as you creep around page so his EMP field doesn't get you. After climbing a small ladder at the end of the corridor, walk onto the lift, crouch and jump to the Mid Level roof below #2 on the map.
From here you have a safety choice. As you can see the gray aliens walking below, you can head west along the south wall until you get to the map point labeled "Bot", hack the security computer terminal there to release it, and watch as it takes out the grays to be safer... then backtrack to the roof above #2 to continue. Or skip that part and continue on. Don a Hazmat Suit, drop to the door at #2, enter the radioactive room and quickly head east to escape the radiation to #3. Around the corner from the dead mechanic's body at #3 is a datacube with the Coolant Control room at #5, 2242. Head back out through the radiation room to #4 on the map, taking out gray aliens or cloaking as necessary.
Below you can see the many animals roaming the Lower Level. Just below you and a little to the right is a map point labeled "Bot". It's another security bot you can activate by hacking the computer terminal near it. I suggest doing so simply to clear the level for more breathing room, and then tossing a LAM at the UC door on the Lower Level (or bypass it), turning the dial inside to block any more respawns.
When ready, head to #5, the Coolant Control door and enter the 2242 code to open it. In my game, for some reason the code did not work so I had to multitool-bypass the door to open it, but your mileage may vary. Once inside, open the next door to see an MJ12 commando patrolling ahead. Sneak in and to the right behind a control desk. The switch on the desk causes flames to shoot out between you and the commando, enabling an alternative route to defeating the enemies here; simply lure them to you, crouch behind the desk and flip the switch as they get to the flame spouts.
To your left as you enter is a ladder up to a catwalk with troops, and ahead is a pipe system you can drop down into, and take the ventilation shaft all the way to the west room near the stairway to the north. Your goal is the #6 coolant control switch, so use one of these alternative methods and hit the Flush System switch at the west end when ready.
Tong will tell you to get back to the Reactor Lab in Sector 3, so let's go there now. Use the lift at #7 to take you up to the Upper Level, backtrack past Page, the alien room, your birthplace, into sector 3, up the stairway at the west end all the way to the top, take the ladder to the first part of Sector 3, out of the Aquinas Hub and go back into the Reactor Lab. Once here, beware of the two gray aliens you may have not killed before on your first trip, don a Hazmat suit, and drop into either the right or left reactor. At the bottom of each reactor is a Failsafe Switch you'll need to push, taking the shaft between the two reactors as necessary.
Head back up the stairs into the Reactor Lab, up the ladder into the control room at the back, and hit the first Ion 1 switch on the left. After that, hit Ion 2, and then ready a gun or prod. The mechanic as you've noticed doesn't like you tampering with the controls. Hit the Ion 3 switch and he attacks. You can prod him prior if you like. Once all three are functioning, simply press the Engage switch on the main console to overload the reactors and witness one of the three endings to Deus Ex. Congratulations on beating one of the best games of all time!