What was reported as using QT was Budgie 11 which was never going to be in Solus 4 anyway. An they're going GTK with Budgie 11 now regardless.
Plasma is not going to be released with Solus 4
If we look at:
eopkg info budgie-desktop
Name : budgie-desktop, version: 10.5
But 10.5 isn't actually an officially tagged release yet. There may be some stuff being held back from what will be the official 10.5 release of budgie in Solus 4, I don't know 🤷♂️
Version numbers are just milestones so for example we know gnome 3.30.x that people wont shut up about is coming in Solus 4.1 as explained here https://getsol.us/2018/10/27/in-full-sail/
I'm sure some are so eager for the latest and greatest of the gnome shit pile who will give Solus a try once 4.1 is out. They know what to keep an ear out for.
Relevent to future of Solus:
But these are just goals not set in stone time frames, as always with Solus, its ready when its ready.