As you know, the red receiver in C-4 level "Oubliette" is a red herring. The "correct" solution doesn't need to use it at all. However, you can actually use it to open the gate:

To achieve this, first you have to place a connector in "Armory" at the right corner of the purple gate:

Now go back to "Oubliette", there should a tiny area allowing the beam to pass through the entrance:

This is the most annoying step. Notice that there is a small window on the wall that lets you see the energy gate:

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Yes, we are going to make the beam passes through this small window. Because of the height of the window, first we have to place a box at the exact point that lets the beam passes through the entrance and the small window (the intersection of two lines). You should use first-person view when placing the box:

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After it is done, place the connector on it. you should be able to see the connector while jumping in front of the red receiver (using first-person view):

The last step is using the recorder. You will need to:

1. Take the platform and stand next to the ledge for ~20 seconds.
   
2. Run to the red receiver and adjust the position until you can see the connector in first-person view while jumping.
   
3. Stand there for ~10 seconds.
   
4. Stop recording.
   
5. Take the connector when the replay starts.
   
6. Make it connects to the ghost connector and the receiver, then place it on the ghost platform.
   
7. Take the box and wait in front of the energy gate until it opens.

Use first-person view while recording:

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Place the connector on the ghost platform:

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The beam can pass through the small window:

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Wait the gate opens: