It’s no trouble to run docker and different containers on an Raspby 4 with 4GB. I’m currently running containers for openHAB, InfluxDB, Grafana, Frontail & Homebridge and use around 30% memory while the average CPU load is way below that.
I would recommend to just try it - if you are already set on using a docker setup you can easily use additional Pis in the future if you have so many different containers that you want to use more machines…
My start into docker with openhab was this post and the linked github repo: https://community.openhab.org/t/the-openhab-2-5-0-update-process-my-experience/