For Cubase 4: This worked on el Capitan; I am running it with no problems.
https://www.steinberg.net/en/support/knowledgebase_new/show_details/kb_show/cubase-studio-40-installer-for-mac-os-x-105-leopard-windows-vista/kb_back/2025.html
However using this:
The installer package stops and reports that there is no software to install.
Is Halion3 no longer supported for newer Mac OS?