You should not do that. Yannick has told me that the sitemap-4seo.xml is a virtual URL. It needs to fall through to the catch-all rule in the .htaccess file which loads Joomla and tells it to handle the request.
All you need to do is make sure that you have xml, xsl and xslt in your “Frontend file types allowed in selected directories” setting in the .htaccess Maker.
Yannick gave me a copy of 4SEO, I have only tested with Joomla 4 though and you are on Joomal 3.10. If this doesn't help please let me know.
Nicholas K. Dionysopoulos
Lead Developer and Director
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