[MrJohz]

It's hard! I only had two users consistently (myself and a friend, and occasionally guests but they taxed the system a lot so it wasn't very often), but I still even up having to fiddle around with a lot.

* As someone else said, overclocking helped, and I had a reasonable passive cooling case to help there.

* I used Paper instead of the normal Minecraft server, and I ended up spending a decent amount of time optimising the configuration. Paper by default comes with a bunch of optimisations, I enabled some more, although I also disabled some that were interfering with the more technical areas of Minecraft that I enjoy more.

* Whenever things started lagging, I went on a killing spree for our main farms, and that tended to work well enough. Most of our contraptions were turned off by default, or designed not to be too laggy. I also restarted the server every night, which worked reasonably well as a sort of ultimate GC.

If I were going to do it again more seriously, I'd probably get a cheap mini PC and use that instead, but for what it was - me and a friend rediscovering Minecraft after having not played it probably in about 5-10 years - the pi4 held up pretty damn well.