Enter in front of you and go up the ladder on the left to reach the roof. Kill the two enemies throwing Molotov cocktails and go back downstairs. To get past the two flamethrowers you will have to move very carefully, using Grant preferably. The person on the right will tell you that you need to destroy an illusion-creating device hidden in a rock wall to continue. Exit north.
All you have to do now is go north and kill the enemies you encounter. Maybe use Max or Nick to try to hit them from a distance. However, you will have to pay close attention to the Neo Knights, who will push you back if they spot you, risking to make you fall. Then position yourself so as to interpose a precipice between you and them, so you can attack them from a distance, or use Grant's cans. As you can easily guess then, wooden suspension bridge are very precarious: cross them quickly, without ever stopping and taking advantage of Grant's agility. After advancing for a certain distance, you will come to an area with rock walls on both sides. It will be right after the second vertical suspension bridge and you will find three Jack Stilettos there. At this point, smash the bottom right rock face with Grant's punch and press the button to deactivate the illusion-creating device. If you don't, you will find that as you proceed, the same scenarios will repeat themselves indefinitely.
The scenery will change and you will find yourself in the snow. You now have the choice between two paths. In the first case, advance north, avoiding touching the barbed wire. You will notice that the snow will slow down your movements. To cross the collapsing bridges, you will have to use Grant and stay on the edge, where there is no snow, or you will fall before reaching the opposite bank. You will come to an area where there are many enemies throwing Molotov cocktails. To get through, you'll have to use some cans from Grant and / or some disguise from Nick, or you won't have a chance to survive. Immediately after the second crumbling bridge there is a life saver, you will need it. Carefully take out the enemies with the machine guns and enter the tunnel to the north. You will find yourself in a room where you will have to avoid being seen by the cameras. The person inside will tell you that you need to find two switches. One is in the next room to the north. Turn it off and go up the ladder to the east. You will return to the open, near the enemy base. Advance through the snow killing enemies and without touching the electrified fences until you reach the main gate.
The other way to get here is to enter the room in the rocky wall to the west immediately after deactivating the illusion device. You will come to an underground passage with blue walls. To reach the east staircase, you need to detonate a bomb from Max against the wall in front of it. It is not strictly necessary to continue, but it will make things a lot easier for you. The staircase will lead you to a room with an orange panel. By destroying it, you will turn off the light upstairs. Go back and continue north. You will encounter many enemies with the machine gun, but if you have turned off the light, they will not be able to see you and you can kill them very easily. When you come to a fork in the road, go east, opening the iron curtain. A little further on you will find a life saver. Then have Grant open the electronic gate and climb the ladder behind it. You will arrive in front of the main gate. This second route is probably easier, but now you'll have to go east and then north to reach the ladder and go down to the room below (you will need a bomb from Max if you come from here), where you can turn off the switch that controls the gas.
At this point, however, go to where the electronic gate described above was. Take the life saver nearby if you haven't already obtained it, then this time go through the iron curtain at the top right. Now that the gas is off, you can get inside without any problems. On the left you can take a case. Then cross the iron curtain below. If you want, you can detonate a bomb from Max to knock down the wall and create a link with the area to the south. Then go up the ladder on the left. You will return to the open. There is a Neo Knight nearby, it is advisable to use your secondary weapons to kill him before he can push you against the electrified fences. Go south and then east until you get back to the main gate. This time, go inside and reach the building on the left, killing the enemies along the way. You'll find a life saver behind a couple of lasers, grab it if you need it.
Go back and this time enter the building at the top right, whose entrance is guarded by two enemies with machine guns. Ignore the person inside and exit north. Continue to the left, killing the enemies and enter the first entrance you will encounter. By avoiding the steam jets and killing two enemies, you can destroy an orange panel, which will allow you to disable all cameras and laser sensors in this level! Then go back and cross the building you entered earlier, ignoring the person inside. Now enter the building to the south-east. The person inside will tell you where to get the ID card. Be careful, there is a floor pit in this room, right between the orange panel and the gate. Then go back outside and this time enter the central building, the one in front of the gate. Cross the room, reaching the upper right staircase. If you have deactivated the cameras as advised, they will not harm you. However, there is a floor pit to avoid, just to the right of the second box. Upstairs, if your informant said "left", talk to the person inside to get the ID Card, otherwise ignore him. Climb the ladder to the south to get to the roof. Here you will find a life saver and a case. Follow the path in reverse until you return to the outside. Now go back to the building at the top right. If your informant said "up", finally talk to the person inside to get the ID card, otherwise go north.
Now reach the westernmost entrance. There are a lot of SMG enemies nearby, so be careful. Inside, go south and after eliminating some enemies, turn off the switch. Now go north and go through the ID checkpoint. There is a life saver immediately after, if you have turned off the switch you can take it without problems. Then go up the ladder to meet the boss.
Compared to the boss of Area 3, this one is a trifle. The only difficulty lies in the fact that you will not have to hit the hostage he holds close to him. If this happens, it will be you who will suffer the damage, which will be curtailed by your life energy. For the rest, the boss will move quickly around the room, where there are crates and barrels, shooting you when he sees you. For combat, you should take advantage of Grant's agility. Try to guess the movements of the boss and position yourself behind an edge. When the boss approaches, hit him with a fist, being careful not to hit the hostage, then move away immediately and repeat the procedure. Hit the boss 10 times to kill him and free the hostage, who turns out to be Shannon, Doctor 0's secretary.
Enter the next room and destroy the orange panel to free Doctor 0. Talk to him and he will tell you that to foil the terrorists' plans you have to play a game against the computer and make it understand that there are no winners in the war. He will also give you the access code: MTKN. Then follow the doctor and his secretary to the roof. A helicopter will arrive and take you away and you will have completed this area as well.