Lylamwyn examines the four keys. Two keys have a symbol associated with the Air Temple. Two keys have a symbol associated with the Earth Temple.
When Lylamwyn presses the two earth keys together or the two air keys together, they remain attached for fifteen minutes until they repel each other.
Pressing one of the air keys to an earth key didn't seem to have any effect.
But when Lylamwyn pressed the combined air keys to the combined earth keys, then all four keys stuck together. And this time the keys did not come apart after fifteen minutes.
Lylamwyn casts the spell Identify on the combined keys to find out more about them. His spell informs him that the keys were designed by the cultists to open the doors of the island in the middle of the lake. Two lesser keys combine to make a greater key; a greater key is necessary to open the corresponding door (the earth keys open the Earth Door, etc.). Combine two greater keys and you have a Master Key which can open any of the doors (Fire, Earth, Air, Water, etc.).
Interogations of the prisoners from the Air Temple reveals the following information:
The cultists worship the Elements, Elementals, and the Elder Elemental Eye. The rulers of the cult live in the center of the lake.
There are four temples in the Crater Ridge Mines (Air, Earth, Water, Fire). The Fire Temple is reputed to be the most powerful of the four temples.
There are four bridges from the Mines across the lake to the Inner Island. Each bridge is guarded on the Mine side by forces belonging to one of the elements. The other side of each bridge is guarded by a Door of the corresponding element (Fire Door).