How to figure out which USB port goes to which USB controller?

[unman]

But as near as I have been able to determine, lspci shows four controllers, yet just one of the four controls ALL of the plugs on the outside of the box. I can’t tell which one, but they all seem to group together somehow in sys-usb. Of course that could just be the (useless to me) mapping that goes on within sys-usb.

This really requires manual testing, but you can automate it to some
extent.
Create 4 new USB qubes, each one allocated a different controller.
Create a script in dom0:

qvm-shutdown sys-usb
sleep 10
qvm-start USB_A
sleep 10
qvm-shutdown USB_A
sleep 10
qvm-start USB_B
...
...
qvm-start  sys-usb

Insert a USB device to the port(s) you are investigating.
Run the script.
(You may need to configure strict reset.)
Watch the popup windows identifying the associated devices.
At end, you return to current state.

If you insert a different type of USB device to each port, you can map
individual ports…

It’s quite possible that only one controller governs all your USB
ports: that’s hardware design.