KF5-using apps currently integrate very poorly into existing KDE Plasma 4 systems. They don't pick up key visual settings, and because of System Settings 4 and 5 not being co-installable in many distros, the user cannot manually sync them. I think it would be reasonable for the plugin to try and load KDE 4 settings when apps run in Plasma 4. Reproducible: Always
Qt5 applications automatically use Plasma 4 settings (at least font and palette), so the code to read those settings from Plasma 5 configuration files should simply be disabled when running on Plasma 4. In other words, when not installing the integration plugin, apps look fine (except having boring file dialogs, and missing icons here and there :)
Nobody cared enough to change this and now plasma 4 is pretty irrelevant. Closing this report as it's not getting any changes anyway.