Hi Gutshot
Error page belongs to Joomla template you're using on your site, if you want to edit this page with T4 Page Builder, you can try this way:
1) Create an article for error page and edit this article with T4 Page Builder (make sure that that T4 Builder Article integration plugin is installed on your site)
2) In the error file in template folder, you can redirect error page to an article: https://docs.joomla.org/Archived:Creating_a_Custom_404_Error/en and you can link to article created above.
Regards