With the Unlimited license you can use 1 Master plus 1 for failover node in cluster within the same license.
If you would like to use 1 master node and 3 failover nodes you have to use 2 seperate license keys in the same version (example: 2 Unlimited licenses).
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.