Graham Harman untangles the fourfold.
It is actually not so difficult to discover which two dualisms are in play here. One opposition recurs throughout Heidegger’s philosophy so repetitively that it often seems like the only idea he ever had: the distinction between a thing’s shadow concealment and its explicit appearance. This is also known as the temporal interplay between past and future, or between the equipment that silently functions and the signs and broken equipment that show themselves “as” what they are. Even in the fourfold of 1949, it is quite easy to split up the terms along these lines. Ever since the essay on artworks in the 1930s, Heidegger used “earth” as a term for mysterious concealment that withdraws from all appearance. By contrast, “sky” is defined in terms of specific visible examples such as the cycling of the seasons and the course of the planets and stars.
It is just as easy to classify the other two terms. Heidegger tells us that “gods” are never visible, but merely hint, making it clear that gods belong with earth on the side of concealment. Meanwhile, “mortals” are defined as the ones capable of death as death, putting mortals on the side of clearing or revealing, due to the role of the explicit as-structure here. Mortals and sky, then, are terms of “future” or of the revealed realm, whereas earth and gods belong to “past” or the concealed realm.