How can I specify which MariaDB instance to launch in a Galera Cluster
According to the output from mysqld_multi --help:

--wsrep-new-cluster  Bootstrap a cluster.


So on the node you want to bootstrap, I'm assuming you should be able to do: 

mysqld_multi --wsrep-new-cluster start 4


And then start the instances on the other two the normal way, assuming all the instances have the necessary config files.