How can I specify which MariaDB instance to launch in a Galera Cluster

I have several MariaDB servers, each with multiple instances running. I'm trying to figure out how would I bootstrap a Galera cluster but specify which instance I want to launch i.e: node1, 2, and 3 each has instances 1-4 running via mysqld_multi, each running on their own port. I want to use instance 4 on each server for the cluster, so I shut each one down. On my test cluster (which has a single instance of MariaDB running) bootstrapping is simple: start it on the first node with galera_new_cluster then bring up the other two servers. How would I do the same thing but specify GNR "4" to bootstrap the cluster?