In an ASM (Automatic Storage Management) setup on a four-node cluster, a database instance on any node can indeed be a client of an ASM instance running on any node, facilitating flexible and efficient storage management. Additionally, a database instance uses an ASM instance as an I/O server for handling I/O requests to diskgroups, ensuring optimal I/O performance. Moreover, an ASM instance and database instances can coexist on the same node, allowing for resource sharing and efficient utilization of hardware resources.