This question is mostly addressed to Wolfi323, who generously maintains the "Branch project for KF5" repository for modern openSUSE releases. This way it becomes possible to install KDE4 side by side with Plasma5. I managed to that on my Tumbleweed system, but I've got few small issues:
- are there any working weather plasmoids that work for KDE4?
- is it possible to get the KDE4 applet for NetworkManager working these days?
- have you or anyone tried pairing your smartphone with the older KDeconnect 0.8 in KDE4? Does it work these days?

Kind regards.