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I am not going to discuss whether it would make sense to use 4 manager nodes or not… I would use 3 manager nodes + a worker node instead. I described why 4 manager nodes has no advantage over a 3 manager node cluster.
You can use node labels use the node labels in your placement constraints to pin services to specific nodes. The forum search should yield plenty of examples for how it’s used.
You last question shows that you didn’t understand what I wrote about unhealty/healty nodes and the quorum. A turned off is treated like an unhealthy node. Deployed Containers of swarm services will keep running, even if the cluster becomes headless. They just won’t restart if a service dies and needs to schedule a new instance.