A 4xx status code is an indication that a request has failed due to an internal server error. This can be caused by various factors such as invalid user input, database corruption, or a malfunctioning application. When encountering a 404 (Page Not Found) error, it's likely because the requested URL was not found on the website. However, the absence of a page at this URL is often due to changes made to the website's structure or content. In some cases, the server may return an empty response, which can be confusing for users. This type of error typically indicates that the server cannot find a specific resource. The 400 Bad Request status code is often followed by additional information in the response body, providing more context about what went wrong and how to correct it. For example, if the server returns an empty response, it might provide a message such as "The requested URL was not found on this server."