"Error 4xx" refers to a category of HTTP status codes that indicate a client-side error when accessing a web page. These errors occur when the server cannot process the request due to issues like a malformed URL or a request for a non-existent page. Common examples include 404 Not Found, which means the requested resource is unavailable, and 403 Forbidden, indicating that access to the resource is denied. These errors are important for web developers and users alike, as they help identify problems with web requests. Understanding 4xx errors can assist in troubleshooting issues, improving user experience, and ensuring that websites function correctly.