Access the red key (and blue key) on PSX Final Doom level
4, Combine:

When you first arrive on this level, you'll appear in the
southwest corner of a large courtyard at the south end of
the map.
Proceed northeast to a hallway at the east side of the
courtyard, and follow it north to a door, and go through.
You'll enter into the south side of a square room.
A stairway winds along three sides of this room.  Take the
stairs up to the northwest corner, and open the door that
leads west, onto a long, raised walkway.
On the walkway, turn right and drop down to where you'll
see another door at the bottom of some steps.
Go through this door and turn right.  Drop down into the
room and pick up the blue key.
Go back out through the door of this room, and go up to the
top of the steps.
Step back onto the long walkway you were on a moment ago,
and go back east, to the door that leads back to the square
room with the winding staircase.
As you step through that door, turn left and open the door
in front of you.
Step north through that door, and onto a wide platform that
overlooks part of a river of green slime.
To the northeast you'll see a radiation suit down in the
slime.  Jump down and grab it.
Follow the river west, until you come to wall in front of
you.  Along the face of this wall are very narrow steps
that lead up and to the left.
Go up the steps to a platform at the top, and use the blue
key to open the barriers there, revealing a teleport pad.
Step onto the pad, and you'll materialize on top of a small
pedestal in the northwest quadrant of the map.  This is
where the red key rests, and you'll be standing on it.
Simply step off the pedestal to automatically grab the key.