How would the HD Guide appear on an old 4:3 HDTV

I don't have an old 4:3 HDTV but I'm just curious how for people with old HDTV's that weren't wide screen how the the guide would appear. If you set the guide's output to 1080i would the guide display in letterbox? If you set the ratio to 4:3 but 1080i would the guide display out of shape or would the graphics not display in hd or is it possible to view the guide in full HD everything but in 4:3.