Various internet research, which is why I've been skeptical. I also spoke with a local contractor who indicated that he thought it would require 4 supplies, but didn't give a size and said he would need to see the space to confirm. What do they look at when visiting the space? I may be able to describe or provide pictures.
Up to now this air has been heating and cooling the rest of the second floor. I've attempted to attach the second floor plan below (space in question is called "Unfinished"). The other three bedrooms each have three supply lines (all 6" flex duct), which I also thought was a little much, but I'm no pro.
Thanks for your help.