I need to change the default paper size from "letter" to "A4"; to achieve this, I changed the region settings in the Clock and region setting from the control panel. I am seeing the page size as Letter even if I restart the PC. Is there any other way to change this?
You should check your NON-UNICODE language settings:
Use the same setting as you use with 'Standards and Formats' which will probably be English (UK), and the result of the next printer installation should be a default A4 paper size. Unfortunately, you have to reinstall all printers after you changed the NON-unicode printers setting to automatically let them use the default A4 paper size.