Nefesh HaChayim, Reb Itzele's Long Annotation

And based on what was previously explained, the four supernal elements are [emphatic] the tzellem and the spiritual model by which will be brought into existence the details of a person’s limbs/organ per the forms of the three vessels: brain, heart and liver, they being the soul-Neffesh, soul-Ruakh, and soul-N’shama. And they will cause in the observable world the activities of thought, speech and action, per the supernal Will. And they are drawn from the four letters YHV”H (blessed be He).