Hi
Any idea what's going wrong and how to fix
The fix is not in Joomla or in 4SEO.
Your server is misconfigured, likely your .htaccess directs any request for a file that does not exist to Joomla, which is why you see a Joomla 404 (whether it's 4SEO showing the 404 content or Joomla does not matter, it's still a Joomla 404).
Looking at your URL, are you sure the MyApi case is right? this is a bit unusual.
And then of course, there's file access permission: the webserver process must be able to access that folder and file.
Lastly, web server block for /MyApi must be configured to use index.php as index file.
Best regards
Yannick Gaultier
weeblr.com / @weeblr