So, I'm reading the third part of the Leaky Cauldron/Mugglenet joint interview with J.K. Rowling and after all the other squeeful things I see this:

JKR: Probably. I hear you. It is the tradition to have four houses, but in this case, I wanted them to correspond roughly to the four elements. So Gryffindor is fire, Ravenclaw is air, Hufflepuff is earth, and Slytherin is water, hence the fact that their common room is under the lake. So again, it was this idea of harmony and balance, that you had four necessary components and by integrating them you would make a very strong place. But they remain fragmented, as we know.

Oh joy, oh bliss! I have notes written down from two years past using the four elements in a post-Hogwarts fanfiction. It makes me happy to see the four elements used here, by Rowling, even if she hasn't consciously employed the metaphor in the books, they seemed obvious from the four houses, their colours and attributes.

The four elements: air, fire, earth and water; corresponding to the four seasons and the four humours (sangiune, choleric, melancholic and phlegmatic), are wonderful topos of philosophy and magical theory.

Empedocles, a fifth century B.C. philosopher, put it this way in Tetrasomia:

Now hear the fourfold roots of everything:
Enlivening Hera, Hades, shining Zeus
And Nestis, moistening mortal springs with tears.

The four elements, the four humours, the four seasons, the four gods: all are required to keep the divine balance. And if the balance is challenged: chaos erupts. The balance is maintained and symbolized by the worm Ouroboros -- a potent symbol of the Gnostic tradition and the alchemical philosophy.

Ooo! It has me all twitterpated, it does. My story idea takes flight, anew!