Looking at doing this with physically separate PCs seems daunting. Even you look at used i5/i7 systems with low-end dedicated graphics you're probably looking at a minimum of $200 per PC, closer to double that if you want anything new or with decent specifications.

Monitors would be another discussion - easiest may be to just get a group of 4, cheap monitors for all your subs, or they make large monitors that can split screen to handle quad inputs. I'll look into other options for managing a 4-way split screen, but its perfectly doable in hardware as well as software.

Hardware Keyboard Multicaster:
http://www.vetra.com/845utext.html For keyboard control, my 1st thought is to look at something like this; 1 keyboard input, with multiple simultaneous outputs. This site makes versions for 2-8 connections, some on USB and some on the old-school PS/2 style keyboards. Its not particularly cheap option - $400 for the 5-port USB version, and then you still need 5 separate PC's to run WoW on.

Hardware programable KVM:
I need to find the make/model we have at work, but at it's core it looks similar to the multicaster above, but its generally for using your 1 keyboard and mouse to jump between different systems - 1 at a time. But you can do the switch without leaving your keyboard - I think the default is alt-F1 for PC1, alt-F2 for PC2, etc. But I'd assume this could also be macro'd. And if you want all your monitors active at one time, just don't hook them um through the KVM - wire them directly. If this could be macro'd into your standard attack sequence it may be workable - KVM's are generally cheaper than the multicasters above. (Or maybe someone could look at programable KVMs and see if there's a way to program them for multiple simultaneous input.)

VMWare:
https://www.vmware.com/products/workstation-pro.html.
This isn't technically a "hardware" solution, but VMWare is pretty close to it - you still have all the pain of managing separate PC's, but those PCs exist only within software - all running on your host PC. For those who have already invested in a big beefy PC, VMWare may work. There are issues with GPU support from within a virtual machine (it generally doesn't work well). But I read that the 3000 Nvidia GPUs are supposed to have better VM support.

Mouse support across multiple systems:
https://www.microsoft.com/en-us/down....aspx?id=35460 For mouse support my thought is not to run the mouse through a multicaster/KVM, but instead run something like mouse without borders or synergy (https://symless.com/synergy) to let your single mouse move across the different PC's - 1 mouse, no cursor or click duplication at all. We've used similar systems at work on servers before - there's generally some noticeable lag when switching between systems, but it may be tolerable for those few times when you need mouse control on a separate toon.