The Campaign Builder's Guild

Actually, in some variations of the greek ideas, there was another element, sometimes called quintessence (or "fifth element") that was identified as something close to what we'd call "life force".

Just saying.

ok, I think i'm just nuts here, but here goes.

Fire
Water
Earth
Air

Law/Knowlage
Chaos/Darkness

Life
Spirit/Energy

Balence

yes there is 9 of them.

now sound would fall under air, lighting would be a Energy/Earth mix, Poison would be a Darkness/Earth mix, ect.

My problem with the Classic 4 is that Earth is too broad and Fire too narrow.  Earth tends to be a grab bag for anything made of wood, flesh, or stone.  I generally equate bone and metal with stone.  Fire should probably include Lightening somehow, or there should be an Energy element.

I've created several elemental systems over the years.  Only one had ever been my definitive system, but that's my personal system which I rarely give out.  Here are some others:
--(for a dragon-based story I did once):
Creation
Light
Fire
Lightening
Air
Water
Stone
Life
Shadow
Destruction

--Sasam elements (and with their corressponding concept associations):
Air: associated with grace, speed, being untouchable, going with the flow, ubiquity, being far reaching, flying (both metaphorical and literal), vapidity
(Phoenix Knight) aware, excitable
Water: associated with mixing, adaptability, power, change
(witchhunt) danger, unpredictability
(Luminous Crayon) wealth, life-force
(the_taken) unrelenting force, absolution, eternity; River and Bay for wealth and travel, Lake and Coast for serenity, healing and rejuvination, the Deep Ocean for age, eternity, wisdom, death, and mystery
(Phoenix Knight) resilient, unyielding, steady
Lightening: associated with speed, surprise, power
(Phoenix Knight) mentally quick, hyperactive, excitable
(Natural 20) being energetic
Stone: associated with stubbornness, old age/wisdom, knowledge
(Natural 20) tradition
(Phoenix Knight) strength, stamina, grounded, hearty, courageous, determined, slow to act, level head
Wood: associated with life, strength, knowledge
(witchhunt) friendship, love, kindness, resilience
Flesh: associated with life, vulnerability, youth
Fire: associated with warmth, destruction, sight, consumption, prudence
(Luminous Crayon) change, courage, brash/recklessness
(Phoenix Knight) strength, aggressive, tenacious, fearless, arrogant, confidence, passionate, intense
Shadow: associated with protection, danger, the inner mind, mystery, deception

I like to use the five Chinese elements (earth, fire, water, wood, and metal) because I once read in a book that each one represented a concept of material or energy rather than just the thing named.  It even explained how each element overpowers another.

Very simple system (based on states of matter/energy):
Sky (gas)
Sea (liquid)
Land (solid)
Spirit (energy)