netplugin | Container networking for various use cases | Networking library

taken from the docs:

Subnets:

Must be large enough to accommodate the nodes, pods, and all Kubernetes and Azure resources that might be provisioned in your cluster. For example, if you deploy an internal Azure Load Balancer, its front-end IPs are allocated from the cluster subnet, not public IPs. The subnet size should also take into account upgrade operations or future scaling needs. To calculate the minimum subnet size including an additional node for upgrade operations: (number of nodes + 1) + ((number of nodes + 1) * maximum pods per node that you configure)

Example for a 50 node cluster: (51) + (51 * 30 (default)) = 1,581 (/21 or larger)

Example for a 50 node cluster that also includes provision to scale up an additional 10 nodes: (61) + (61 * 30 (default)) = 1,891 (/21 or larger)

If you don't specify a maximum number of pods per node when you create your cluster, the maximum number of pods per node is set to 30. The minimum number of IP addresses required is based on that value. If you calculate your minimum IP address requirements on a different maximum value, see how to configure the maximum number of pods per node to set this value when you deploy your cluster.

this means for your case you need 30*4 + 4 = 124 ip addresses needed minumum for this to work, but keep in mind if you would want to add 4 node and upgrade it wouldn't work. if you would to want to scale to 5 nodes it wouldn't work. Also, what's the point if such small subnets? you don't pay for the subnet size, so making them reasonably big is not an issue

means you need /25, technically. 128-4 (reserved by azure) = 124 ;)

Reading: https://docs.microsoft.com/en-us/azure/aks/configure-azure-cni#plan-ip-addressing-for-your-cluster