You could add the "rock core" ppa and install qt4, as I did for Ubuntu 20.10.
and add qt4 using sudo apt install qt4.
This will enable Fulcrum to build with qt4. Note that qt4 will exist alongside qt5. Applications that need qt5 will still use it.
Note that I had another application (elmer fem) that had a graphical interface built using qt4. I was able to use cmake-gui and set the cmake value
"DESIRED QT VERSION" to "5", and it built properly. However, I don't know if you could do this with SCALE.