@Arash 

In your 4 node NiFi cluster, what value do you have set in the "nifi.remote.input.host" property in the nifi.properties file for each of the 4 nodes?  It should be the FQDN for each node and not be the same value on all 4 nodes.

Form the host where MiNiFi is running, can all 4 of those FQDNs be resolved and reachable over the network?  If not, MiNiFI RPG is only going to be able to send successfully to one FQDN it can reach.

When the RPG is started it reaches out to the URL configured in the RPG to obtain S2S details from the target host.  That target host collects the host details for all currently connected nodes in the cluster and communicates that back to the client (MiNiFi).   If all 4 nodes report the same configured FQDN in the "nifi.remote.input.host" property, then client only knows of one FQDN to which it can send FlowFiles over Site-To-Site (S2S).

To improve redundancy in the RPG, you can provide a comma separated list of URLS in the RPG configuration so if any one node is down, the RPG can try fetch S2S details from the next host in the comma separated list.

Hope this helps,

Matt