There’s very little confirmed about 4.0. The only things I can guarantee are Yae Sakura, Edo Star, and Stellaron Hunters having a role—everything else is too far off.
About Sunday: Amphoreus is still there, and after the Amphoreus arc, he should leave alongside Black Swan.
Cyrene’s role: Seems to be designed as a support, but March 7th’s role is still unclear.
About March 7th SP: Aethiphia’s sales expectations have been set very high, just below the main promotional unit―something to look forward to.
About 3.8: I heard that Shaoji made a special patch for Firefly because he think Fireflys character development is weak.
About Phainon: Q: Hmm, among Sparkle, Bronya, and Sunday, is there any that fits with Phainon? A: All three fit well with him.
They aren’t upgrades over these three. I’ve thought about it, and it seems like Cerydra and Delos aren’t upgrades over any existing characters right now.