Graham Harman on the ontological difference in the fourfold.
[I]f there is one axiom of [Heidegger's] thinking that never varies, it is the constant opposition he draws between absence and presence, veiled and unveiled. And his choice of wording in Bremen makes it perfectly clear that earth and gods are both terms of concealment. As early as the artwork essay in 1935, the task of earth is to withdraw from all access, and the same holds true in 1949. As for gods, they are said only to “hint” rather than revealing themselves. By contrast, Heidegger tells us that mortals are linked with the as-structure of explicit visibility. And as for sky, it is clearly a matter of specifically visible entities, as opposed to the ceaseless withdrawal of earth.
So much for the first Heideggerian axis, a division repetitive and profound enough that it would make Parmenides proud. But there is also a second axis in Heidegger’s thinking that makes it easy to read the fourfold in a second direction as well. In 1949, that axis is a second version of the famous ontological difference, or the difference between being and beings. For there arc two possible ways to read this difference. One is to read it as a distinction between veiled and unveiled, absent and present, withdrawn and cleared, implicit and explicit. But it can also be read in a second sense as meaning that being is one and beings are many. And for Heidegger, this second axis repeats itself on both levels: veiled and unveiled. For on the concealed level “earth” is always read as a single, unified force. And the same is even more obviously true on the visible level of “mortals,” who are mortals not by experiencing many different things, but by encountering beings as a whole: namely, in the form of death “as” death. The opposite is true of the other terms. On the concealed level Heidegger says “gods” rather than God, not as some gratuitous slap at monotheism, but to show the contrast between the singular earth that withdraws and the plurality of cryptic messages that the moment of “gods” represents. And on the revealed level, the menagerie of items included under “sky” contrasts with the singularity found in earth and mortals. In other words, the fourfold can be viewed as the intersection of two related but variant senses of the ontological difference: veiled vs. unveiled, and one vs. many.