To be very specific, MacOS doesn't even pretend to maintain application compatibility as new versions are released. In the “good old days” it was requirement of responsible operating system developers that existing applications continue to run with no modifications on updated or new versions of an existing operating system. Even Windows which used to be pretty good about maintaining application compatibility from one version of Windows to the next is no longer very reliable in this regards.
For better or worse, if you upgrade computers with a computer that has more advanced technology (such as 4K or 5K very high resolution screens, advanced GPUs, tablets or touch devices, etc.) or that requires a newer version of an operating system, you should also assume that many if not all your applications will require either updates or new versions. When you get into very complex applications, as the Adobe applications are, that touch very many system resources and facilities, the likelihood of requiring a new application version increases significantly. This is nothing that Adobe wants, but is effectively forced on us by the operating system developers.
As Bob Levine indicated, you will need to go back to your previous MacOS version for the CS4 software (at least 8 versions behind the current application versions) to work. Hopefully you have backups!
- Dov