It would be great, if x11-libs/qwtpolar would use multibuild.eclass to support USE="qt4 qt5" both at the same time, similar to how it is done in x11-libs/qwt-6.1.3-r1.
While that would be great indeed, I am not aware that the build system supports parallel install of the headers and libs...
How is this handled for x11-libs/qwt? From the ebuild I would guess that the assumption is that the headers for Qt4 and Qt5 are the same and one can simply override the other during installation...
I just pushed 1.1.1-r2 which removes qt4 support entirely.