Is it possible to use a Mac Pro as four different PC's simultaneously using the multiple display output
Yes, this is possible to do. However, you will probably find that you'll need a lot more than 16GB RAM to make this practical. 

16GB RAM is more than enough to simultaneously run macOS and a virtual machine. But in your case you're wanting to run macOS and four virtual machines. Remember that you will need to allocate both RAM and VRAM to your virtual machine from the 16GB of RAM you have on board.

My recommendation would be that you have a minimum of 32GB RAM installed so as to make the setup workable. Ideally, you'd install more than that. Officially Apple supports 64GB RAM in these machines, however users have found that they work just as reliably with 128GB RAM installed.

If I was going to do this, I'd actually have 64GB RAM installed and probably share one of the VMs with a Bootcamp installation. However, that's just a suggestion.

Either way, you only need to install Parallels once and then use it to create 4 virtual machines. If you do share one of these with a Bootcamp installation, then you'd obviously need a partition for that, but otherwise there's no need for multiple partitions.

In terms of the four displays, you would just move your VMs around to assign each of them to a different screen.

Finally, since you're using Parallels, you could even use it in 'Coherence' mode so you can have any app you're running on any of the screens without having to worry about which VM was running it.