If you check my AO3 or Wattpad version of this story, I made Talent cards for Call of Artemis, Unichord, and Abyssmare, as well as assigning them roles within the D4DR-verse.

CoA is stuck inside a Hope's Peak building (one not being used by Junko's Killing Game for Makoto's class). They are keeping tabs on the Tragedy. As the Ultimate Announcer, Mana in particular does a lot of the reporting duty. The others will later play a peripheral role in explaining the story of Stage IV. Magic will be a part of the plotline, and given this unit's name, there will be references to the real Goddess Artemis with the idea that the unit was created in honor of her.

UC is roaming around Towa City because Michiru is trying to find Shinobu (who is trapped in the Killing Game, unbeknownst to all), Lumina is trying to find her memories (which will connect her to Neo, and to the technological half of Stage IV that will involve finding/building bodies for sentient minds), and Kokoa and Hayate are just out there trying to not die. Their romance will be a minor backdrop of the story in that I want it to be the one healthy, not-depressing part of this saga, LOL.

Abyssmare is with the Future Foundation/overseas American academy, tasked by Ibuki with looking after the Foundation in Photon Maiden's absence, but Abyssmare's selfishness makes them blind to Kazuo Tengan's schemes to push them out of power and take it all for himself. Elsie and Weronika spend all their time fighting, Elsie possibly lying about her Talent while Weronika worries about her siblings. Neo is fixated on being the Ultimate Diva and cares only about power rather than wielding it wisely (which will give her a relationship with Kazuo that mirrors her relationship with Sho in D4DJ). Only Sophia is paying any attention to the larger plot, but she's "Cassandra Truth"-ed by all the others and seen as a nutty conspiracy theorist.

So yes, magic and technology will be remixed in Stage IV to bring back some dead students, but it will be a while until it is posted. Feel free to enjoy the rest of my currently-published D4DR stories and art until then!