Unfortunately due to the whole Qt->Digia movement all the old FTP links are now dead, and Digia (or qt-project.org) doesn't seem to provide the old SDKs as a download. Luckily there are still some mirrors left, e.g.:
This is important for Windows users because Qt 4.6 requires a GCC 4.4 compiler, and finding MinGW binaries this old is next to impossible. The SDK contains the MinGW compiler and can be found in the mirror above. We should put it in the wiki on dsource.