As far as what happened between G4 and G5, here’s a list of points:
*The windigoes haven’t appeared
*The day/night cycle is working
*The three tribes have lost their magic
*The three tribes no longer get along
*Whatever the three crystals are, they need to be together to bring the magic back
*The crystals didn’t show up in G4, so it’s not unreasonable to assume they were created between the generations.
Between the windigoes and the roles the three species performed, I’m willing to accept that magic really was the glue that held the tribes together, and that without it, they would return to their default state of not trusting each other.
I’m going back to one of my earlier theories, which is that one civilization getting to control The Sun eventually drove a wedge between the civilizations. I think the magic disappearing was a side effect of trying to automate the day/night cycle, and the crystals were created to counteract this effect.