Modbus?
I was looking at the same typology kinda (publisher -> server -> subscriber) but having a signal from the mill be the publisher to a "channel" on a MQTT server and m4 being the subscriber (using a PLC script to ping the channel). That way I can control m4 from anywhere there is a Internet connection if I am away from the area. Well, theoretically anyway.
I know, your thinking, why go through an external server for signal processing when I can just send the signal straight to m4.....just trying to look a things from all directions. That's all. I'm really interested in learning more about TCP/IP config & comms with m4. I get where you are coming from. That's neat!