I'm just wondering where the KDE4 application appearance settings are saved? I am specifically talking about System Settings -> Application Appearance -> Widget Style.

I've just compiled KDE4 on one of my systems which also has KDE5 and cannot save this setting. All KDE4 programs have whatever generic theme comes with stock QT4. If I change this setting to Qtcurve, Oxygen, etc. the programs change themes as they should. But if I launch any other program after this, it has the stock theme until I change the theme again. If I log out and back in again, they will all have this stock theme.

I am careful to have NO references to QT5/KDE5 anywhere in any environment variables, /etc/ld.so.conf, and so on when launching KDE4, so I suppose a configuration file is getting in the way. The first time I launched KDE4 some of my theme settings from KDE5 were picked up such as the highlight color of selected text. I know having a separate user account would be ideal for this, but this particular setting seems to be the only thing not working.

It turns out this was (probably) caused by the qtcurve theme that I built for qt5. Apparently this theme has its own configuration files in ~/.config/qtcurve which were being picked up in KDE4 where I selected the qtcurve widget theme, thinking it would pick up the qt4 version. I had to run $QT4PREFIX/bin/qtsetup and choose qtcurve there, and then save this option. Then I could choose any other theme or keep qtcurve. I guess the qtcurve config files explains how my theme colors and so on were already active in KDE4 when I had never set them. Anyway I got it working, and it's interesting to have both usable from the same home folder.