See themes tagged as plasma##majorversion=4 being hidden, except when they
are also tagged as plasma##majorversion=5. This can most easily be done
by launching the plasma theme dialog and opening Get New Themes from there
and turning on debugging for KNewStuff(Core) like so:
`QT_LOGGING_RULES="org.kde.knewstuff*=true" kcmshell5 kcm_desktoptheme`
and then watching the output, and seeing that content is being rejected
by the InequalityFilter for plasma##majorversion when appropriate.