You can't. The way Qt things are done on Maemo means this gets decided when the bindings get compiled/packaged. So, while you can have simultaneous 4.5 and 4.6 support, this would mean ANOTHER set of libraries, imported separately. There was supposed to be a python2.5-qt4-maemo5 package, but considering the imminent release of PR1.2 and lack of testers I never pushed it to extras-devel under that name (it will replace the current package upon PR1.2 release, instead)