A possibly small yet interesting group that caught my attention. As at first thought was reference to the Four Auspicious Beasts, but as I looked more at it, I realized that this might be related to the Kabbalah's Four Worlds.

"The Primordial One appeared to do battle against the Seven Sovereigns and, to that end, created of itself." Is a line that convinced me that it was possible that it was indeed related to the Kabbalah.

The Four Worlds are made of world of Earth (Manifest), Air (Formative), Water (Creative) and Fire (Archetypal).

Istaroth is the Goddess of Time and Wind, which would fit in with the World of Air. Formative can mean something that has influenced on the development of someone or something, in this case, I think it might be possible to connect it to how her existence has influenced the concept of Wind and the sayings within Tyevat. Additionally there is the fact that she could possibly involved with the creation of the Sacred Sakura, another hint of influence. (Albeit this is an incredibly weak statement)

From what I can see, Shade of Life can represent the World of Earth, as she helped the Primordial One of creating and manifesting various creatures, in fact, the small info in the wii about her just talks about her creating life.

Additionally, this connect back to Adam Kadmon, who is not included in none of the worlds, however is sometimes considered the "fifth world" or better yet "the first world" as it transcends from the other ones, this explains the fact that The Traveler goes from world to world, but at the same time is from this world.

A problem with this is the fact that you could argue that this would mean that Aether is also the Primordial One, as this person is the one who created the Four Shades in the story. If the theory of Aether being the Primordial One is still alive, then this could be used as a supporting argument that could be searched further.