The Four Elements that make up the world—fire, water, wind, and mountains—were adapted from classical alchemical theory with minor modifications.
There are actually five elements in Four Elements, namely the four mentioned above plus Element V and Mana. This evolutionary relationship corresponds to Plato's theory of five elements and the four elements of Western alchemy. Over the course of long transmission, Mana (the ether among the five elements) was forgotten, and nowadays it's commonly believed there are only four elements.