Since 4.18, support for 4 and 5 control plane installations is added. But, there is no documentation available for expanding an existing cluster add 4th or 5th control plane node. The assisted installer document [1] does clarify adding 4th or 5th master is supported. 

[1] https://docs.redhat.com/en/documentation/assisted_installer_for_openshift_container_platform/2025/html-single/installing_openshift_container_platform_with_the_assisted_installer/index#installing-control-plane-node-healthy-cluster_expanding-the-cluster

The Openshift documentation only has procedure to add worker nodes into the cluster. There should be a documented procedure to add control plane nodes into the cluster. This can be added in this [2] section of the document. 

[2] https://docs.redhat.com/en/documentation/openshift_container_platform/4.19/html-single/postinstallation_configuration/index#adding-worker-nodes_post-install-cluster-tasks

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The Openshift documentation should have a documented procedure to add 4th or 5th control plane nodes into the cluster as well.     

- This query was raised in forum assisted installer. I got confirmation on the slack thread that this is supported as day 2 operation.