com sudo apt-get install python-qt4 qt4-designer
deve funcionar.
Você pode encontrá-lo se pesquisar com apt-cache search qt | grep designer
:
libqt4-designer - Qt 4 designer module
libqt4-designer-dbg - Qt 4 designer library debugging symbols
qt4-designer - graphical designer for Qt 4 applications
kdesignerplugin - Integration of KF5 widgets in Qt Designer/Creator
kdesignerplugin-data - Integration of KF5 widgets in Qt Designer/Creator
kgendesignerplugin - Integration of KF5 widgets in Qt Designer/Creator
libopenrpt-dev - graphical SQL report writer, designer and rendering engine (development)
libopenrpt1v5 - graphical SQL report writer, designer and rendering library
libqscintilla2-designer - Qt4 Designer plugin for QScintilla 2
libqscintilla2-designer-dbg - Qt4 Designer plugin for QScintilla 2 (debug)
libqt5designer5 - Qt 5 designer module
libqt5designercomponents5 - Qt 5 Designer components module
libqt5scintilla2-designer - Qt5 Designer plugin for QScintilla 2
libqt5scintilla2-designer-dbg - Qt5 Designer plugin for QScintilla 2 (debug)
libqxt-designer0 - LibQxt extensions to Qt Designer
openrpt - graphical SQL report writer, designer and rendering engine
E pesquise mais informações sobre o pacote com apt-cache show qt4-designer
.