The Four Elementals puzzle is in the Plaza of the Sun Hub. In the Obelisk of the Moon a sign post will tell you:
"The Four Elements are needed to finish the Bridge of Stars, only then will the Well of Souls be opened."
In the Bridge of Stars there are four pedestal's scattered around which will give you a messages:
"You need the Element of fire."
"You need the Element of Earth."
"You need the Element of Air."
"You need the Element of Water."
These are where you come back to the place the elements you have found.
The 'Test of Fire' and the 'Test of Water' are in the Plaza of the Sun, where there are two doors one to each test. They won't be open until you have completed other tasks.
Walking through the ornamental key door will give you the message 'Prove yourself in the Test of Fire' . In this area there are a series of moving platforms which you will have to jump onto to press three buttons on the wall. When this is done you will get the message:
"You have proven your worth"
If you go back to the entrance you can get the Element and the message:
"You possess the Element of Fire."
The other door leads to the portal to the Square of the Stream for the Water Test. There is a door leading to a small fountain as you approach the water a message says:
"Prove yourself in the Test of Water"
For the test you have to go into the water and navigate through an underwater maze, the speediest route (and you are holding your breath) is 1 left, 2 rights, 1 left, 2 rights, left and then up. Get the message:
"You have proven your worth"
Go to the altar in the next room and:
"You have acquired the element of Water"
The Element of Air is in the Palace of Columns, you will find a sign post:
"The High priest Coatepec has take the element of Air with him to the other side"
You know need to find your way to the Tomb of the High Priest. There is a maze and moving blocks to negotiate to get to the Catacombs where you will find three rooms in one of them is an alcove with the Element of Air in it, grab it and get the message:
"You have acquired the element of Air."
The 'Test of the Earth' is in the Court of the 1000 Warriors, when you get to the right area you will get the usual message:
"Prove yourself in the Test of Earth"
Go through one of the tunnels with the sliding block and reach the alcove your done:
"The Element of Earth is yours"
Back in the Bridge of the Stars you need to put the Elements on their pedestals; this completes the bridge and you get the message:
"You may now enter the Realm of Death"
Job done.