For those that want 4 relays cheaply using a clone of the Seeed studio V2.0 relay board available on AliExpress here, this is how it’s configured in HA. Simply follow the Arduino instructions to load the Firmata firmware on your board then plug the Arduino board into your Raspberry Pi.
i have a very big house also.
i have 3 mysensors gateways in the moment and will probably go up to 7 gateways.
i have already extended my WIFI with 3 accespoints and will probably need 2 more to have wifi strong enough everywhere.
but if WIFI is strong enough then you can have mysensors gateways there also.
and nowadays you can have sensors and switches directly at your gateway also.
at least you are a lot more flexible then when you need to connect your arduino directly on your RPI.
now you can only connect the relays on things near your RPI.