For example try adding a resource file to your project using KDevelop. Or add it manually to the project file, then load it into KDevelop and do something so that the project file gets saved by KDevelop. Then look at the resulting .pro file.

Another example -- try turning on or off some of the modules from Qt (like gui, xml or whatever). Or try to do it manually in the project file and then open KDevelop and make it resave the file. Then look at it. You can repeat that for every feature which is not present in Qt3.

Third example -- install Qt3 development files along Qt4 development files. Make sure that Qt3 files are ahead of Qt4 ones in your $PATH and then try to tell KDevelop to compile using Qt4 without changing the $PATH.

Fourth example -- did it install Qt4 documentation so that you can use it from KDevelop's integrated documentation browser? Do you see both Qt3 and Qt4 documentation in the tree there?

If you want more examples, drop me a line.

What is more funny -- there is an entry somewhere in the configuration dialog of KDevelop to set the path to qmake. Well... the funny part is, that KDevelop never checks that when looking for qmake. It has some paths hardcoded, so if you have qmake in some non-default location, it won't find it.