If you open the app and look at the topology page (Network) you should see all the discs and how they are connected.
If you are seeing only the set of 3, then the 4th Disc has not installed correctly.
To start again try this,
1. Leave the set of 3 working and connected, to the Hub/Router
2. Connect the 4th disc to the hub using the Ethernet cable power it up and press the factory reset.
3. Wait for the solid blue LED. (About 2 minutes) this will copy the information from the set of 3 to the 4th disc.
4. With the 4th disc factory reset, power down and open the app and add another disc following the App instructions.
Hope this helps