To fix a 404 error on localhost, you can try the following steps: 1. Verify that the file or directory you are trying to access exists in the specified location. 2. Check the URL for any typos or incorrect paths. 3. Ensure that the file or directory has proper permissions for the webserver to access. 4. Restart the Apache server in XAMPP to refresh the configurations. 5. If you use custom URLs or virtual hosts, ensure the configurations are correctly set up in the httpd-vhosts.conf file.