Can you replace font used by Qt app (the one that ships with its own libraries)
 

I've been wondering whether there's a way to set up Linux so it would swap font in a Qt4 app for other, specified one? Especially considering the app ships with its own libraries (I see quite a bit libQt*.so files in the app folder).
I'm looking for a way to do it as the app I use for work has a font that is not very cool to work with on higher resolution screen, and tweaking dpi is not ideal too - a slightly "fatter" font would be better).

I'm not sure what you mean, but you can set the default font of an application with QApplication::setFont() in Qt 4.8 and QGuiApplication::setFont() in Qt 5.7. Before you can do that, you need to have an instance of either QApplication or QGuiApplication, depending on your Qt version.

More information about the QFont class:
http://doc.qt.io/qt-4.8/qfont.html
http://doc.qt.io/qt-5/qfont.html