The meaning is much simpler than that: the fifth refers to Death. Here is what the verse says:
From terror of Brahman fire burns, from terror the sun burns, from terror Indra and Vâyu, and Death, as the fifth, run away.
The verse is listing five things: fire, the Sun, Indra, Vayu, and Death. So all it's saying is that Death is the fifth item in the list.
Adi Shankaracharya says the same thing in this excerpt from his commentary on the Katha Upanishad:
How the world lives from fear, of him, is explained. The fire burns from fear of him, the lord of all; the sun shines from fear; from fear, Indra and Wind; and Death, the fifth, runs; for, if Brahman did not exist as controller of the competent protectors of the world, like one with the thunderbolt uplifted in his hand, their well-regulated activity, as that of the servants trembling from fear of the master would not be possible.
By the way, on a side note, in this verse Yama is actually paraphrasing a quote from this chapter of the Taittiriya Upanishad.