It is only an obligation if one has a garment with 4 corners.
R. Chaim Krauss answered that since it is equal to all the Mitzvos because it reminds us of all of them it is not obligatory in order to remind us of "all" mitzvos , even those which are not obligatory !
Now may be a good time for a gem of Torah study.
www.GemsofTorah.com
