Not sure how well thought out this theory is, but I think it's fun.
Also, I'm new! Hello!
I think the sun in SP4 may be one of the following
1) the grave of a highly invested person
2) the site of a shard splintering itself
3) both?
We've seen significant light given off by highly invested entities before, and we know the light is a sort of investiture because it seems to be powering the jet turbines that are keeping everyone alive.