Plasma 5 is a whole new version and won't co-install with Plasma 1. Nevertheless some parts of runtime should co-install so apps using kdelibs4 can run under Plasma 5.

not co-installable

these are new version of the KDELibs 4 equivalents from kde-workspace and won't co-install, no change expected

libksysguard
ksysguard
kinfocenter
kmenuedit
kwrited
systemsettings
khelpcenter
powerdevil
plasma-desktop
plasma-workspace

yes co-installable

kwin is now co-installable
baloo libraries are co-installable but not the binaries (although they are drop in replacements)
kfilemetadata co-installable library version