with 4 more nodes set up and avoiding "split brain" problem mention the no. of master node as
discovery.zen.minimum_master_nodes in configuration file of elastic search.
Also for 4 node set up it is better to use no of master node is 3.
you can calculate the best no. of master node in your set up via following formula : N/2+1
for you configuration it is good to mention "discovery.zen.minimum_master_nodes:3"