"The Fourfold" is an ontological illustration of the world in which man, or Dasein, dwells - as used by the philosopher Martin Heidegger in his later works, such as Building, Dwelling, Thinking, The Thing, Language, and Poetically Man Dwells. The Fourfold seeks to poetically describe Being and its relation to man, which has been covered through the development of Western metaphysics and the death of the gods. Thus, with the Fourfold Heidegger attempts to overcome the nihilism of metaphysics where man once again sees the "question of Being" as an issue and dwells within Being's presence.
*Note*(For those not acquainted with Heidegger this description may seem like gibberish.)The fourfold is the coalescence of earth, sky, divinities, and mortals. The united interplay of this foursome is the world in which man dwells. The strife of earth, sky, divinities, and mortals causes the Open of the world to occur. Earth is the active unfolding of phusis, the formation and growth of rock and sea, the journey of the continents, the rise of plant and animal from the Earth's supple life-giving ground. Earth is self-sustaining and secluding. Sky likewise can be described as the unfolding of phusis, the forming expanse of clouds, the gusting of wind, and the flight of the sun and moon onto the depths of aether. Divinities are messengers of godhead, made known in their concealedness in the sky and on the earth. Divinities are the means by which Being is revealed to man, much the way ancient poets were inspired by muses. Divinities are the measure of man to his world - thus poetry, unadulterated, invokes divinities. It is important to note that Heidegger uses divinity as an ontological term, referring to no particular deity, but rather to an aspect of Being's truth being made present to man in the strife of the Fourfold.
When one speaks of earth, sky, mortals, or divinities, individually, one speaks of the oneness of the Fourfold - because the mention of one element necessarily refers to the other three, for they are inseparable and essentially one. Thus it is inappropriate to speak of mortal man without already picturing him standing there in a field or on a hillside above him the expanse of the heavens, receiving sky and earth, awaiting divinities.
The implication: our current epoch is godless, in that it lacks a measure for man. Metaphysics and modern technology warrant that man is the measure of himself, and how reckless he has thus become! It is clear that Heidegger sees poetry as the way in which to create measure for man through allowing Being to be present to man through invoked divinities. Here is a quote from Thinker as Poet (p.3) to illustrate this point:
"We are too late for the gods and too early for Being. Being's poem, just begun, is man." (Martin Heidegger, Thinker as Poet as translated by Albert Hofstadter in the compilation Poetry, Language, Thought (1971)
*Sources for this node are the mentioned texts by Martin Heidegger.*