Technically, as of 2018/03/02, both Qt4 and Qt5 are still supported and we have not updated the build instructions on the wiki.
Rational: For now, it allows us at Kitware to keep using the latest version of Slicer while our customers to transition their Slicer-based applications to Qt5.
That said, this will change very soon (few weeks) as we will remove the support for Qt4 from the code base and support building only against Qt 5.9 and beyond.
Before doing so, we will create a tag to identify the last revision working with Qt4.
To summarize, the next steps are:
- Complete the transition of daily Linux build to use Qt5
- Finalize update of build instructions on the wiki
- Remove Qt4 support from the code base