In regards to the Four Tones bit, for those that wish to reread, it's Chapter 60: Come and See. Chapter starts from Adolin's POV and then switches to Venli where she views the chasmfiends singing with four tones.
I personally hate the idea of a 4th Shard being on Roshar all this time. However Valor is a dragon and Wit says they are "tricky lizards who hide where you least expect them" in chapter 21 of Wind and Truth.
But there is a lot of weird stuff about Natanatan/Narak/The Shattered Plains.
Site of the fourth moon/dead moon crash sight. At some point before the Shattering Roshar had a 4th moon. Shattered Plains seems to be where it fell. But my gut says fragments of the moon probably scattered all over Roshar. See point 6
In the past when Odium and Honor clash at Natanatan. Their brief fight seems to create anti-light. My question is does this happen whenever a Shard clashes or is it a result of the moon fragments underneath Natanatan?
The metal under Narak, fragments of the dead moon, can hide things from the Shards. There were people that were previously hidden from Tanavast that he sees while Natanatan is being destroyed. He calls them Watchers so maybe the Sleepless.
Tanavast concludes this metal is something "greater than aluminum"
Natanatan is destroyed in a cymatic pattern becoming the Shattered plains. All the dawn cities cymatic patterns of their own. Are moon fragments under Kholinar, Vedenar, etc?
Odium ends up placing his perpendicularity there? I'm not exactly sure on this point. It seems like that's what it is in Venli chapters and the he places it there after Ba-Ado-Mishram attempted to usurp his power. And at the end of the book the pool fills with the Black and Blue light and Venli hears the Rhythm of War/Retribution's tone.
I'm beginning to wonder if these moon fragments being "greater than aluminum" is something like Duralumin. Acting as an investiture amplifier or something. Might explain why when the Highstorm and the Everstorm initially clashed it was so violent.