Door 4 was one of the two numbered doors the group first encountered. It was a sturdy, metallic double-door, seemingly heavily bolted, with a pair of handles protruding from the center.
After a brief introduction of all the characters, the 9th Man held Clover hostage with a pocketknife, and with Ace's help (4+1+9 =14 =1+4 =5), opened Door 5 and entered it alone. Upon finding that the DEAD would not stop his bracelet, the 9th Man cried for help, which ultimately, was not possible. 81 seconds later, he died behind Door 5.
The group opened the door without entering to inspect what happened. After a lengthy explanation and discussion, the group finally decided they would be entering the numbered doors.
By default, Ace, Snake, Clover, and Seven organized themselves into a party to go through Door 5 (1+2+4+7 = 14 = 5), leaving Santa, Junpei, June, and Lotus to go through Door 4 (3+5+6+8 = 22 = 4), unless Junpei decided to insist he wanted to go through Door 5, in which case, the group would have to reorganize itself.
Behind Door 4
Upon entering the door, the party found itself in a very long hallway, lined with doors on both sides. The DEAD was, however, nowhere to be found, causing the group to worry and start trying to open every door in close proximity, with no success. After a few tense moments, Junpei noticed the small blinking red light of the DEAD at the very end of the hallway, and the party ran towards it, arriving just in time to deactivate their detonators. The danger averted, the group finally took notice of their surroundings.
In front of them stand three doors, the largest of which was locked, but also engraved with the Mars symbol, and was clearly the way forward, while the two smaller doors were unlocked (the only such doors in the whole hallway) and led to twin 2nd class cabins, which together constitute the next escape room. The group then split up into two to explore those rooms, in the hopes of finding the Mars key to open the door.