The following skull can be picked up in Forge mode on the map Citadel
From the middle of the map, go towards the blue side, and turn left in the hallway. You should see an arch as you turn. If you follow the wall to the top of this arch on the right hand side wall, you should be able to spot the skull sitting on top of a small beam.
Exit forge mode and make sure you pick up the skull.
It can be seen in the video below, and I have included a timestamp of its exact location:
] - Skull 4 - Citadel