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?
If you are on macOS and use brew you may have installed qt already using brews installer.
In that case you already have QT Designer installed even though it is not mentioned anywhere.
You can find it here: /usr/local/Cellar/qt/
So to place it in your local apps folder you can create a symlink to it like this:
ln -sf /usr/local/Cellar/qt/5.11.1/libexec/Designer.app ~/Applications/.