Нello everyone. I installed Plasma 5. There are two problems:
1. I can not customize the look of KDE 4 applications.
2. In the file selection dialog open folders with one click instead of two.

Are there ways to solve these problems?

Look if you can install "systemsettings" along "systemsettings5" to enable customization of the 4.x applications. BY file dialogs, you mean KF5 file dialogs, or from the old 4.x apps?

In order to put the package with the application settings for KDE 4, I have to remove Plasma 5.
But, apparently, must be some configuration files that you can edit manually?
The problem with dialogs relevant to applications kf5.

Systemsettings 4.x and KF5 can be installed side-to-side: openSUSE does that, for example. This looks like a packaging problem and you should enquire with your distribution.

I used qtconfig-qt4 to set a widget style for kde 4 apps.