Be aware that every member of your group needs to have this mirror quest done in order to be able to progress further inside this instance.
If someone in your group is unsure; have them check their quest journal, or right click the mirror and see what option shows up. If they can select "Enter the Mirror" they can enter to the next floor. If they can select "Gaze into the Mirror" they need to get and finish the mirror's quest.
Once you have "entered the mirror" the only way to get back down is to evac or die. Clicking on the mirror you exit from will zone you out to The Shimmering Citadel, possibly locking you out from the instance.
To get to the fourth floor, you must complete the scarab ring events in the three rooms. Once you've done that, you will be able to enter the central door and click the mirror. On this floor all the doors, except the doors to the side rooms, opens randomly and stays open for about 10 seconds before closing again. You might have to wait anywhere from a few seconds up to several minutes before a door close by actually opens.
All the roaming djinns will eventually repop. The minotaur statues don't.
If you just click on the mirror it will default to remember the vision which will pop the orbs, but if you do not right click and choose gaze into the mirror and get the quest - the red orb will not be clickable.
To get into the room with the fourth floor mirror, you first need to do 3 ring events. Once they are all done you will be able to open the doors to the room.
Northeast room ring event (first you can get to from the exiting mirror)
3 sets of emerald scarabs popping along the edges of the room, before the final set of 2 glimmering cobras close to the doors, one 64^ and one 64^^^ linked together.
Southeast room ring event (second you can get to from the exiting mirror)
3 sets of sapphire scarabs popping along the edges of the room, before a final 64^^^ glimmering cobra in the middle of the room.
Southwest room ring event (third you can get to from the exiting mirror)
4 sets of ruby scarabs popping along the edges of the room, before the final set of 2 glimmering cobras close to the doors, one 64^ and one 64^^^ linked together.
Right click the mirror and gaze into it to get the quest.
Right click the mirror and select "Remember the vision" to pop the orb (clock) you need.
All around on this floor orbs will now pop along the walls. A random one will be glowing faintly red and be clickable.
Find the clickable orb, examine it to wind the clock in it and get your update.
Go back to the mirror, right click it and select "Enter the mirror".