I just installed Qt 5.4.1 on Windows 7. And there is no QtDesigner. Also there is no QtDesigner in MaintenanceTool.
How can I install it?
You can install and find QT Designer as follows (Windows environment):
C:\Qt\5.8\mingw53_32\bin\designer.exe
UPDATE - there is an easier way to install QT Designer without downloading GB's of data from QT:
Install the latest version of "pyqt5-tools" using pip install pyqt5-tools --pre
The "designer.exe" will be installed in ...Lib\site-packages\pyqt5_tools