Not for clothing only
Yesterday we learned that a four-cornered head-dress is not obligated in tzitzis, because it does not qualify as “your garment”, which implies a garment whose primary purpose is to cover the torso. In a similar vein, there commonly existed a four-cornered garment which was worn around the shoulders and torso; however, its purpose was to be available for functional use, such as wiping off perspiration, wrapping up merchandise, protection from rain, etc. Such a garment also, although worn on the right part of the body, and also used at times as clothing, is exempt from tzitzis because it is not used exclusively as clothing.
Which kind of four-corners?
When the Torah stipulated a four-cornered garment, it had in mind two in front and two in back, at approximately the same height. Accordingly, even if a garment technically has four corners, if they are all on the same side (e.g. as some jackets do) or two are at the top and two are at the bottom, it is probably not obligated in tzitzis. To be sure, a halachic ‘sheilah’ (query) should be asked in cases of doubt. Alternatively, a person can remove the doubt by rounding off one of the corners!