Compatibility
CKEditor 5 has been available since Drupal 9.3.
5 is a major rebuild, so there are substantial differences in the codebase and compatible plugins.
Many modules have since been ported, but you may consider sticking with CKEditor4 until you're prepared to make the jump.
https://www.drupal.org/project/ckeditor
New features
Multi-user collaboration (premium)
Output to PDF/Word
Developer tools
Themes
Updating to CKEditor5
If you have a site with heavy use of CKEdtior 4, you need to check which modules are compatible.
If you have a minimal setup, you can just switch your text format's editor to CKE5.
You can create a new text format for CKE5 and start using it going forward. There is no easy way to bulk switch content to use the new format, so adjusting the existing one is preferred.