The fact that the page titles can be changed in edit mode is not so visible anymore

But with CK4 we have two visual styles for editable text, one with borders and one without.

You're right, but it doesn't bother me that much.

Could it be handled by an automatic style switch when CK4 is the default?

The edit mode/form (be it standalone or inplace) is not supposed to care / be influenced by the actual widget (CKEditor / BlockNote) used to edit a given field (e.g. the document content). That widget is provided by an extension that can be uninstalled. The edit mode shouldn't provide styles for widgets that might not be present. The widget (CKEditor in this case) can of course load some custom styles that affect other fields from the edit form/mode (like the title field). It's not super clean, but acceptable I guess.

I wouldn't change the title field style (it doesn't bother me, and I'm not convinced users don't know it can be edited), but if you and the others think the border is necessary then I'd put this in the CKEditor styles, somewhere in CKEditor.EditShet's SSX.