Ninja
Hi Pankaj,
I completely understand that T4 template is for the front-end. Let me then explain something new. You take the editor.css file from Joomla 4 Cassiopeia template css folder and copy it into templates/t4_bs5_blank/css folder.
Any new classes you make or modify in editor.css will show in the TinyMCE editor as classes from the drop down - like in my image above. TinyMCE editor has a function to look for editor.css in front-end template folder - which will override the core Joomla.
Generally we will just add the class name to editor.css and use something like inherit. Just so it is there. This will generate selection in TinyMCE. Then, you can use custom.css to style this class - so your custom CSS is in one place for editing.
No modifications to admin template are needed. Just need to make sure this editor.css file does not get removed when updating T4 Blank.
Note: This works in Joomla 3 also.