Would having 4x 4TB WB HDD's connected to the MINI via powered USB be too much stress on the MINI ?
I don't see it being an issue at all, with the exception of the 2011 Mini having USB2.0 ports. Not that this is really an issue either, unless you end up saturating the bandwidth on the ports, and even then, it might just be slower. Probably would never happen just from a few streams to Plex.
Multiple users simultaneously accessing a single 3.5" spinner for video streaming seems like a bad idea.
I don't do it anymore, but I used to use a single 3.5" HDD for Plex without issue.
Currently, I am using my old Mac Pro, with two large HDDs in a SW RAID0 for my Plex media, a SSD for the OS, and a single large HDD to back up the SSD, Plex Media. I also keep a collection of old OS versions on the large single SSD for troubleshooting, using old SW, and the old Disk Utility app.
I plan on switching my Plex server to my M1 Mini when the high end Minis come out, and retiring the Mac Pro for just backing up my media server, troubleshooting, and old SW/OS access.