TL;DR at the bottom. Sorry for the length and formatting, first post here.
Before I begin, I would like to present a WoB that I think makes a lot of our theorizing so far incorrect, that many either are not aware of or are forgetting.
This to me would very clearly indicate that with our newfound knowledge of there being four dawnshards (confirmed by the Sleepless, who also mentioned they aren't from Roshar and are very much cosmere aware,) one of them is different from the other three. Makes sense, right? So how could it be that it would be different, but still part of a matrix like what is being suggested?
We know that one is Change. So is that the different one, or one of the set of three? (That might be a good question for Brandon sometime.)
For now, I'm going to work with the assumption that it is one of the three. I posit for the next Dawnshard the command: Live. These are supposed to be very primal, correct? So we have Change, Live, and something else, with an additional odd one out. But, neither of these seems to be able to bind, which is a big point of what we are expecting based on evidence. But real quick, I would argue that Live is what Hoid held for a time, as he can't harm or eat meat. I'll have to go back through the WoBs and not during my reread of the books all the occurrences of Hoid and food but for the time being I think it was Live. Think about it as a primal force, Commanding to Live. You can't kill or harm if your primal command and directive is to LIVE, as a command for creation. It's meant to be tricky to grasp but I argue for it nonetheless. (Also, investiture left alone becomes sentient, Soo....)
So what else is primal, and we also need to keep in mind that one is different and one can bind. I don't have a solid argument on this, but it does seem pretty accepted that Unite or something akin to it is likely the next. So for now, let's go with that one as it is the idea of bringing together, which could involve literal matter in a physical creation, as well as more abstract such as joining peoples. So now it would seem we have our set of three Dawnshards, being Change, Live, Unite. These don't appear to overlap to me, feel free to correct me.
So what is the last Dawnshard? No idea. It's different somehow though, according to the big man himself. But how is it different? If they all relate to creation, then it's unlikely, in my reasoning, to be destruction. Brandon has stated MANY times that investiture and matter and energy are all the same in the Cosmere, and that everything gets recycled back into the system (or at least isn't destroyed, hence Night blood absorbing and retaining investiture without it being destroyed or leaving the system completely, which although not explicitly stated seems pretty obvious to me..). So destruction is off the table by this. So what if the last is something more like Challenge? It is thematically different, and also seems to fit with how the Cosmere works and the greater plan that seems to be going on, with world's eventually coming together and all that fun stuff. Compete could be another possible Command, but TBH I really don't like it and it doesn't feel right. But Challenge? Maybe.
Also, Rysn does mention that the sun mural is broken into four MOSTLY even pieces that are further broken. There isn't an overlap between the four sections. The word 'mostly' to me indicates an oddity, potentially the one of four being different somehow. However, could also be artistic and not literal.
Adonalsium was not angry at the shattering, but was confident, resigned and was accepting of what was happening. This was not some being/entity caught by surprise. I would argue that the light was a remnant of Ado, and that the guardian of Ancient Sins is indeed Chiri-Chiri, and the ancient sin was the shattering of Ado. Seems clear, but why and how? How is Chiri-Chiri a guardian of that?
The Dawnshards aren't a matrix like has been posited abundantly. I would suggest a set of three and one, with the three being:
Change, Live, Unite (join, bind, etc. You get the meaning)
The one, I think could be:
Thanks for reading my theory!