You can port really big apps from Qt3 to Qt4, using the qt3to4 tool. What you will end up with is a program that depends on the Qt3Support library. Your final goal should be to eliminate dependence of the Qt3Support library. That is the hardest part of the porting. The qt3to4 tool is basically a search-and-replace program. You'll have to do quite a bit of work yourself before the result will build, link and run.

There is an archived presentation of Qt3 to Qt4 porting at: http://www.ics.com/products/qt/webcast.html