Trolltech did not make it easy to find the installers for the GPLed Qt 4. This article provides direct links with tips and tricks.
This is the downloads page for the X11 Edition of the GPLed Qt 4. According to Trolltech's site, there are packages for Conectiva, Debian, FreeBSD, Mandrake, Red Hat, Slackware and Novell/SuSE (however, urpmi (i.e. Mandrake) does not find Qt 4). Since the configure script works well and is easy to setup, most developer should be able to build their own version. Do not forget to add database plugins in the build process.
Since I'm not a mac user (and PearPC is too slow) I will simply refer to the Mac OS X Edition download page. The file "...contains the complete source code of the library and extensions with full reference documentation. It compiles on Mac OS X." according to the page.
The win32 editions works great. It not only provides a nice installer that installs Qt, but the installer also downloads and sets up the compiler environment (mingw). Simply visit the Win32 Edition download page and download a file with the -mingw suffix. At QtForum, there was alot of noice about instability and performance issues, but this seems to be F.U.D. since my installation has worked without a flaw this far.