The Four Primary Elements are a way of analysing bodily sensations, to distinguish them from mental feelings. And also as a description of energy - bodily and mental.
Einstein's formula for energy-mass equivalence was borrowed and brought here as a metaphor for interchangeable usage when viewing subjective sensations. The choice of terminology being at the discretion of the practitioner: whatever gave rise to the clearest comprehension at any one time.
The Earth element is pressure, whether solid or space, [there being enough energy in one cubic centimetre of vacuum to power the city of New York for one year: zero point energy]; the Air element is vibration or movement; the Fire element is heat, temperature, combustion or digestion.
Water is a holding energy, spatially binding the other three elements together, so as to maintain the perceived object's form. Whilst supporting or enabling the existence of the other three elements, it is only inferred, or clearly comprehended. It may be regarded as the medium within which the other three elements serve as it's constituent characteristics.
Thus water is understood as the primal requisite for energetic existence [life], though, in and of itself, is entirely undetectable. However, at the disappearance of the water element, the other three are said to revert to their primal condition, as in the dissolution of the body at death.