take the qt5 extensions out of your systemfolder. otherwise you can't install qt3/4. after the installation put your qt5 extensions to the extensionoff folder and make a new configuration, one with qt5 on and on with qt3/4 on. you can't have them both at the same time, so you have to decide befor starting your computer what you want to do.
hope that works without problems,
Fragicula