we have started removing Qt4 support from the master branch. From here on, master branch will require Qt5.
Why the change?
For a while, we were trying to do the same as with Python: Support both versions. We even got pretty far; the gr-qtgui widgets seemed fine across versions. However, we had a lot of ancillary apps and modules, mostly in Python, which were never working with Qt5. Making all of those work with both versions of Qt would have been a lot of work.
On the other hand, Qt5 has been around for so long, its support is ubiquitous across all major distros. And unlike Python, most GNU Radio users aren't integrating GNU Radio into applications that specifically require Qt4.
What's left to do?
There are still a bunch of applications that were only written for Qt4. All of those need to be ported to Qt5. In some cases, it's as simple as s/Qt4/Qt5/. In other cases, APIs need to be adapted. If someone wants to help with this, all help is welcome!