They are called mortals because they can die. To die means to be capable of death as death. Only man dies, and indeed continually, as long as he remains on earth, under the sky, before the divinities. When we speak of mortals, we are already thinking of the other three along with them, but we give no thought to the simple oneness of the four. This simple oneness of the four we call the fourfold.
Mortals are in the fourfold by dwelling.
But the basic character of dwelling is to spare, to preserve. Mortals dwell in that they preserve the fourfold in its essential being, its presencing. Accordingly, the preserving that dwells is fourfold. Saving does not only snatch something from a danger. To save really means to set something free into its own essence.