What does the 4XX HTTP status code range refer to?

The 4XX HTTP status code range indicates client-side errors. This means the problem lies with the request sent by the client (e.g., a web browser or search engine bot), not with the server's ability to fulfill a valid request. A common example is 404 Not Found, where the client requested a resource that doesn't exist. "Redirects" are represented by 3XX status codes (e.g., 301 Moved Permanently). "Server-side errors" are indicated by 5XX status codes (e.g., 500 Internal Server Error), meaning the server encountered an issue while trying to fulfill a valid request.