How do I determine which IP I should use for my MariaDB Galera Cluster?
First, you should have 3 nodes in your cluster (or an odd number), not "3-4", so to always know which is the majority group and avoid the split brain problem.

You can specify any of the three node IPs as the database is replicated in all nodes.  However, to make the system resilient you should have a Virtual IP address that allows you to reach any of the nodes.

In my system, I have a S9S ClusterControl with two HAProxy load balancers associated to the same Virtual IP address.  This VIP is the address that clients use to contact the cluster.