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.
The error indicates that, for some reason, your remote device is not "seeing" the Fiddler proxy. While there are many possible reasons why that happens, the most common are:
- The proxy is not properly configured on the target device.
- The Fiddler host Is not in the same local network as the target device.
- A security tool (like a firewall, antivirus, zero-trus tool, administrative policies, etc.) limits network access.