Elasticsearch cluster of 4 nodes has "master not discovered exception"

Elastic Stack Elasticsearch
18

root@SERVER:~# curl -XGET 'http://localhost:9200'
{
"name" : "SERVER",
"cluster_name" : "servername",
"cluster_uuid" : "QyHQj6TXTzG9ZYjC0B5ZCA",
"version" : {
"number" : "6.1.2",
"build_hash" : "5b1fea5",
"build_date" : "2018-01-10T02:35:59.208Z",
"build_snapshot" : false,
"lucene_version" : "7.1.0",
"minimum_wire_compatibility_version" : "5.6.0",
"minimum_index_compatibility_version" : "5.0.0"
},
"tagline" : "You Know, for Search"
}