I want to install zigbee2mqtt on one instance of home assistant, but since the building has 4 floors and thick overlaps between the floors I need to connect 4 instances of zigbee2mqtt to one home assistant. What is the best way to do this?
Drop that idea and use more Zigbee-Devices on mains. They act as routers/repeaters.
Either light bulbs or smart sockets are fine. Or dedicated ones, like those from IKEA
I have exactly your scenario. My central HA instance runs on a RPi4 with 8GB. It hosts one Zigbee2MQTT instance and the MQTT broker, both as HA add-ons. On the other floors I have RPi 3bs with Raspberry Pi OS and the standalone version of Zigbee2MQTT. This works like a charm and integrates perfectly into HA. The external Zigbee2MQTT instances only need to connect to the broker. They then create HA entities through MQTT discovery just like the add-on instance on the main Pi does. I guess RPi zero Ws would do the job too and would save energy. But I prefer to have my Zigbee hubs attached by LAN cable for stability. All in all a very recommendable setup.
I just came across another very interesting option:
There seem to be an option to run Zigbee2MQTT on the Sonoff Zigbee Bridge (ZBBridge). This is an ESP8266 based hardware at rather low cost, which should also minimize the power consumption.
The chipset seems to be supported in the latest versions of Z2M.
But there are warnings to not connect the bridge over WIFI because of stability problems. All in all this sounds brand-new and immature. But it is an interesting new solution.