seems 4.0RC2 is missing some kde packages so it cannot be installed according to the guide
https://www.qubes-os.org/doc/kde/ package plasma-workspace requires kde-setting-plasma but none of the providers could be installed -- You received this message because you are subscribed to the Google Groups "qubes-users" group. To unsubscribe from this group and stop receiving emails from it, send an email to qubes-users+unsubscr...@googlegroups.com. To post to this group, send email to qubes-users@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/qubes-users/CAKYr3zzX0BYkyYZv%2BpM7cwdG_TgOVdxnx7MxLfD0%3Dr2pfo-Y1Q%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
