The task involves a little bit more than just mounting five Pi4s in a chassis and calling it done. There is an Ethernet switch connecting all the modules to the network, and each Pi has a comparatively bulky SSD drive + enclosure attached. By far the most annoying part of the assembly is the power supply and distribution cabling, which is further complicated by remote controlled power switching relays (one of the computers is dedicated to power management and can shut the other four modules on and off).
this one which contains a whopping 750 Raspberry Pis. Have you ever used a colocation service, and if so, did you use a DIY or an off-the-shelf server?