Arduino Web Server and Relay Module
Documentation on the controller you are using is pretty scant, but it looks like you just need to make a GET request to the URI corresponding to the action you need to take.


  http://192.168.1.4/30000/00 : Relay-01 OFF
  
  http://192.168.1.4/30000/01 : Relay-01 ON
  
  http://192.168.1.4/30000/02 : Relay-02 OFF
  
  http://192.168.1.4/30000/03 : Relay-02 ON
  
  http://192.168.1.4/30000/04 : Relay-03 OFF
  
  http://192.168.1.4/30000/05 : Relay-03 ON
  
  …
  
  http://192.168.1.4/30000/14 : Relay-8 OFF
  
  http://192.168.1.4/30000/15 : Relay-8 ON
  
  http://192.168.1.4/30000/41 : Enter
  
  http://192.168.1.4/30000/40 : Exit
  
  http://192.168.1.4/30000/42 : Next Page
  
  http://192.168.1.4/30000/43 : Next Page


For example If I wanted to turn off relay 3, I would simple make a GET request to http://192.168.1.4/30000/04 and close the connection. Example code:

IPAddress server(192,168,1,4);
EthernetClient client;
client.connect(server, 80);
client.println("GET /30000/04 HTTP/1.1");
client.println("Connection: close");
client.println();
client.stop();