Okay... first time really theorizing around here... hope I don't embarrass myself too badly. Here are my notes.
Well, what seems pretty important to me are a few words:
Thing one: "There are four whom we watch.... The world can change.... One of them may redeem us, and one of them will destroy us." (Emphasis mine)
So, multiple people- and they aren't participating. They're watching. Don't know what this implies, but it sounds important.
And then there's the may redeem us and will destroy us. Who does "us" refer to? Does "us" refer to the people of Roshar, or just those who are watching (possibly the spren)? As we all know, Pattern is constantly talking about how eventually Shallan will betray her oaths and kill him. That's a point towards a spren's voice here. (When looking through previous posts just now, I saw that NavySealsGuy said something to this effect.)
Then there's "Men seek that which is lost. I fear the struggle will destroy them." This can be interpreted to mean the speaker isn't human, but it's debatable. This reinforces the point, to me, that the person is just watching.
Aaand wild blind jump into complete blind speculation!
What if the speaker is the one who wrote the second of the Letters? It would explain why he's just watching. The tone seems about right. Still, wild guess. Thoughts?