Hot to allow using all HTML tags in CK Editor 4

You need to add these settings where you have initiated editor.

CKEDITOR.replace('ckeditortextarea', {
        allowedContent: true,
        fullPage: true,
    });

or by this method..

var config =  CKEDITOR.replace( 'editor1' );
config.allowedContent = true;
config.fullPage = true;

This will allow using all cods in HTML mode, I used it to edit email template HTML files with inline CSS and it is working fine now.