Did some testing around Joomla 4 environment today. In Pre-Update check, JCE was marked as potentially problematic. However I did the migration and there were no problems. I tested the editor from the new environment, opening articles etc. and everything seemed to work fine. Even the custom css files I had added for rendering template classes (Gantry 5) worked.
But when I went to the JCE Control panel, clicked on Editor profiles and then the active profile, I got:
1264 Out of range value for column 'checked_out' at row 1
Thought you'd like to know.
So far I can't enter any of the profiles' configuration.