So I was wracking my brain because Arcadion 4's theme reminded me of something and I finally pinned it down:
listen to the first 12-ish seconds of the theme: https://www.youtube.com/watch?v=_htiXfLqXxU
now, listen to this timestamp, 32 seconds in of the four fiends theme: https://youtu.be/9fBOQdbWcRY?t=32

Sounds familiar right?
I now have a crackpot theory that this might be intentional foreshadowing.
We fought the FF4 four fiends in endwalker, yet the OG four fiends, Tiamat, Lich, Kraken, and Marilith haven't been properly fought. Yes Tiamat is an NPC, and we fight a kraken a few times, but not the bombastic classic FF bosses.
Now, as many of you know, Dawntrail, specifically the areas of heritage found and solution 9, reference FF9 heavily.
Now, which is the only other mainline final fantasy game to feature the original four fiends as bosses? FF9
I have a feeling either tier 2 or tier 3 of Arcadion may have fighters that use the souls of Tiamat, Lich, Kraken, and Marilith, or at least their counterparts from the unlost world. In fact it is probably more likely they won't be directly referenced by name but by thematics and design.