Addon Domains and SubDomains are pointed to sub-folders of your hosting account and can have their own error pages. Create error pages you would like and upload them to the appropriate folders:public_html/domain1/error404.html
public_html/domain2/error404.html
public_html/subdomain/error404.html
Now you have to direct error requests to these pages. You can do this by creating .htaccess text files and placing them in the addon/subdomain directories with the following directive:
ErrorDocument 404 /error404.html
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