This section identifies pages that return a 4xx client error code.
A server will return a 4xx error when the client has made an invalid request. The reasons for this include lacking authority to access the page, using a request method that is not recognized or allowed, bad request syntax or (most famously) the requested page does not exists.
Like 5xx errors, pages that return 4xx errors cannot be accessed, crawled or indexed by search engines or visited by users. This means your pages won’t appear in search results and users will leave your site without engaging or converting.