kde-plasma/plasma-desktop became stable and recommended to use instead of kde-4. But there is only 4th version of kipi-plugins, no 5th.
kipi-plugins:4 works for any desktop environment. kipi-plugins:5 will enter tree when there is an upstream release.
Not released yet. You can use beta version available in kde overlay.
kipi-plugins-4 works, but the issue is that it depends on kde-apps/kde4-l10n which blocks kde-apps/kde-l10n