HI @Calzor Suzay
You wrote:
Can you run multiple MQTT On one RaspberryPi4
What do you really mean with "multiple MQTT"?
MQTT is a client/server protocol.
In order to use it, you need a MQTT server, often called a broker. This server can be installed on the same hardware as e.g. Domoticz.
There exist many different brokers, such as HiveMQ, Eclipse Mosquitto, etc.
A comprehensive list, you can find at
https://github.com/hobbyquaker/awesome-mqtt#broker.
Probably one of the brokers (servers) that is most used, is Mosquitto,
It is sufficient to install one instance of Mosquitto on your host or even in your local network. It is possible to install a second instance, even on the same host, but in this case you need to give it another port number. The default port number is 1883 and if you install another MQTT server on the same device, you have to configure another port number, e.g. 1884.
All other devices, such as the Domoticz "MQTT Client Gateway with LAN interface" are clients (see the name). Also other "hardware" devices, such as Zigbee2MQTT, Shelly MQTT, but also the Desktop/Laptop clients, as MQTT Explorer and MQTTfx are clients. This is also the case with the CLI clients mosquitto_pub and mosquitto_sub, part of the mosquitto-clients package.
You can connect as many clients to your server, as you like (compare it as browser to a particular website).
Als the clients should publish their payloads under a different topic, so that other clients can subscribe to it.
@lwolf says:
They can use the same mqtt server, as the MQTT path used by the plugins is different
The different path, he mentioned, are the different topics. It is true for the plugin, you mentioned, but you should check that if you use MQTT, that the topics are different. But pay attention, that if two Domoticz instances, publish to the same MQTT server, they use the same topic (domoticz/out and domoticz/in). So do not configure 2 Domoticz devices to the same MQTT broker.
To answer your question.
Can they all run on the same box, IP, port etc?
Yes, all "hardware" can run on the same box and, yes that box has got one (wired) single IP address and Probably one (wireless) IP address.
It is not necessary to change the port number, so leave it default 1883.
Your CPU power is more than enough. (I run one of my Mosquitto servers on a Pi 1) and MQTT is specially developed to run on low bandwidth networks.
See:
https://mqtt.org/
From Wikipedia:
It is designed for connections with remote locations where a "small code footprint" is required or the network bandwidth is limited.
Regards