http://ipv4.fiddler:8888/ is the Fiddler Classic's echo service and it will work on your machine only when you have the Fiddler Classic running and using the default port (8888). So, just run Fiddler Classic, ensure you are using port 8888 (in the Fidlder options -> Connections) and open the http://ipv4.fiddler:8888/ - it should work.