Ohh I see. Is the fourth Shard theory in direct conflict with/opposition to the fourth primeval spren theory? I like the idea of a Day or potentially Sea primeval spren, but I think there might be some issues with the idea of a Day spren. Namely, once the Stormfather dies, highstorms on Roshar stop happening. Being so related to Wind, if that’s what happens in the absence of the Stormfather, it would seem like if a straight-up primeval spren died (or whatever else might have happened to it), it would have a pretty noticeable effect on Roshar as well, right? There’s still a day/night cycle on Roshar, so it would seem to imply that, if there were a Day spren, it would still be alive/intact. I think a similar argument could also be made for a Sea spren, though we admittedly don’t seem to know much about Roshar’s oceans. I feel like there could plausibly be something really funky going on that would suggest something wrong with a potential Sea primeval spren if we knew about it.
Also, I’m curious what the other “vanished” Shard is that Taravangian can’t immediately see... although maybe this will become obvious as I read more Cosmere books.