I am currently at Big Sur 11.4. how do I get back to 11.2.1 so Photoshop will function?

You would need a TM backup of before you updated. Then start up in Recovery mode and reinstall the OS using TM and pick the date where you had 11.2.1 (it will show that version in the log). Remember to back up any files before doing this that were changed after that date as TM will erase all that info when reinstalling.

If Photoshop ran on Big Sur 11.2.1 it will run on Big Sur 11.4. What problems are you having? Describe them in detail.

Additinally, download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

Update to Photoshop 2021 v22.4.2 on Big Sur 11.4.

The only was is if you had made a copy of the Full Installer of 11.2.1 to external drive OR if you have a bootable Clone when 11.2.1 was installed. Since 11.4 was released - Apple does not offer and older version like 11.2.1