4xx error status code
To earn an excellent reputation with users and search engines, and a solid top position, all of your website’s resources and links must be error-free.
NOTE:
In some cases (especially with older and slower websites), your pages may return a 4xx or 5xx error simply because the server couldn’t process the request in time during testing. When testing, you can rule out this kind of error by reducing the number of requests per second sent to the server.
Not finding any pages with 4xx errors yourself doesn’t guarantee that users or search engines won’t run into problems navigating your website’s content. Make sure all resources are available and load correctly in the website’s code. Also check the pages on your domain for 5xx errors, and make sure your 404 page is set up correctly.
How 4xx errors affect website optimisation
4xx errors often signal problems on a website. If a URL is incorrect or links to a page that doesn’t exist, the browser shows a 4xx error. It’s important to monitor these errors regularly, look into their causes and fix them! 4xx errors have a negative effect on optimisation and lower your domain’s authority in the eyes of users and search engines alike.