[gentoo-user] What is forcing the qt4 USE flag?

[Nikos Chantziaras]

I have a weird problem. I have an ebuild where either qt4 or qt5 can be 
enabled. They are both disabled by default and I have to choose which 
one I want. The ebuild does that with:

   IUSE="qt4 qt5"
   REQUIRED_USE="^^ ( qt4 qt5 )"

I'm on the plasma profile which enabled qt5 automatically. However, 
portage complains:

   The following REQUIRED_USE flag constraints are unsatisfied:
   exactly-one-of ( qt4 qt5 )

The qt4 USE flag is enabled. I can't see where. I didn't enable it 
anywhere in any of my /etc/portage/* files. If I emerge with:

   USE="-qt4" emerge package

then it works. So I have to explicitly disable the qt4 USE flag even 
though I didn't enable it in the first place.

Can someone enlighten me?