And to apply what you have read, here is the explanation for your cluster.
As you can see here, each node have 1.9 CPU and 3.3 Gi RAM allocatable. If you take a look at free RAM for all nodes you have 3.7Gi which is more than the value allocatable for one node (3.3Gi). But look at free CPU, you have 1.6 CPU free while a node has 1.9 CPU allocatable.
In order to down scale and delete a node, all pods must switch to other nodes and for this you need to have at least free resources equivalent to the node that could be deleted:
- 3.3Gi free RAM. This one is ok since you have 3.7 Gi free
- 1.9 free CPU. This one is not good since you have only 1.6 CPU free
It explains why you have four nodes running and why scale down is not triggered. Hope it makes it clear for you.
Feel free if you have any questions about this resources consumption.