Very helpful. Can you confirm you are extending to all 4 displays and not mirroring on any of them using the wireless HDMI adapter? I can get 3 external monitors but not the Surface (pro 6) which I didn't think was an issue until I realized none of my monitors have sound. I would prefer my setup to look exactly like yours with the surface in the middle as long as each of the 4 screens works independently and doesn't just replicate another display.
Thanks!