QTDIR is only relevant with Qt3. Qt4 seems to rely only on qmake. So the only thing you need to build with Qt4 is a proper installation and qmake in your path. The best proof for that is KDevelop itself -- to compile using Qt4 and having both Qt3 and Qt4 installed it is enough to do (in my case): "PATH=/usr/local/Qt4/bin:$PATH kdevelop".

And what's more -- QTDIR is only needed during compilation (qmake relies on it, I guess)! If you distribute your compiled app, there is no need for that. Normal dynamic linker rules apply.

With Qt4 it seems to be.... emm... different. The path to Qt libraries seems to be imprinted into the app itself, at least under Linux, so as long as I don't move my libraries, I can have them in a custom place (not usually scanned by the linker) and they will still be found (which wasn't the case with Qt3 -- libqt-mt.so had to be in a standard library path).