Find the four torches. These are all in the maze on the sub-level below the 4th Floor, reached by jumping in the hole at ( 54, 289, -3 ) /waypoint 54, 289, -3.
Torch 1 ( 8, 268, 74 ) /waypoint 8, 268, 74
Torch 2 ( 46, 267, -54 ) /waypoint 46, 267, -54
Torch 3 ( 15, 268, -23 ) /waypoint 15, 268, -23
Torch 4 ( -30, 268, 56 ) /waypoint -30, 268, 56
Return to the main 4th Floor level. This can be done either by clearing the sub-level (including Deception) and using the mirror from there, or more easily by using any of the three spiral ramps and unlocking and using the door at the top.
Place the four torches on their respective dias. These are around the circular path of the 4th Floor.
Simply picking up this quest (by standing on the first dias) can update the quest The Tower of Frozen Shadow for the relevant step. For this to work, you must not already have the quest "A Light for the Shadow" in your journal, this quest must not already have been completed by anyone in this zone instance, and of course you must be on this step of the The Tower of Frozen Shadow quest. Note that the update is not obtained if you accept the quest being shared to you.
This quest is repeatable (although it is not marked in the journal as such), but will only give XP (and AA XP) once.