To answer the question of why four of them:

* the traffic from the monitored systems comes with spikes. Looking at netdata graphs, currently the baseline is 600 requests/s, but there is a 2000 requests/s spike every minute, and 4000 requests/s spike every 10 minutes.

* want to maintain redundancy and capacity even when a load balancer is removed from DNS rotation (due to network problems, or for upgrade)

There are spare resources on the servers, especially RAM, and I could pack more things on fewer hosts. But, with Hetzner prices, why bother? :-)