not sure that this will help - but found these relevant forum threads
https://discourse.vidvox.net/t/2018-mac-mini-and-intel-uhd-graphics-630-gpu/517
https://discourse.vidvox.net/t/6-8-hd-outputs-for-exhibition-egpu-options-with-vdmx/876
Sounds like the Mac mini isn’t as bad of an idea as I originally thought (for simpler 1080p set ups), but 4x 4k streams streams w fx will be a lot to process. Not sure of the specifics of the project but if budget permits sounds like a good reason to get a new Mac Pro w Radeon Pro W5700X with 16GB ram. Could be over kill. Think best plan would see if you can either rent or borrow a machine(s) to do testing.