Everything that exits in this material world has a pre-existing counterpart in the non-material world. Hence, I suggest that in the case of Earth, Water, Air and Fire, their pre-existing non-material forms are Aether, Spirit Energy, Vital Life Force and Divine Light. Modern science identifies Plasma as being a kind of fifth element of matter, because it does not quite fit in the other categories. Likewise, some metaphysicists speculate that Darkness may be considered a fifth essence of the non-material plane.
The important thing to know is that, here in our solar system (or Heliosphere), the Sun is the source of the four elements we find on our planet. Likewise, the “Sun-behind-the-sun” is the source of the non-material essences. And each of these components can be found within you, as a physical and spiritual being.
I believe that what is being sought by scientists now is not so much the “fifth element” as the “unified theory.” Likewise, spiritual seekers continue to quest for their own version of a “lost chord.” I suggest that missing element of unification is: Unconditional Love.