Update for Photoshop CS4

The reason CS4 will not work on macOS Catalina is Photoshop CS4 is 32 bit and on macOS Catalina 32 bit applications no longer work.

https://support.apple.com/en-us/HT208436

Apple typically provides updates for two versions back, which means that High Sierra and Mojave will get updates for at least another year, then Mojave for a year after that (given Apple's release schedule of once a year.)

Older applications will not work going forward. That simple. CS4 is no longer supported and will never run on anything past Mojave. You would need to purchase a subscription to one of Adobe's plans to remain current.

Other software such as Microsoft Office is the same- only the current version of Office 2019/365 works on Catalina.

https://www.adobe.com/creativecloud/plans.html

There will be no update for CS4, CS5, CS6, CC, CC2014, CC2015, CC2017, CC2018. You are 8 versions behind, and support for new systems is a new feature, no matter how much people want it to be a free bug fix... CC2019 has had an update and is nearly there. But on a macOS upgrade be sure to budget for all new apps.