The halachah is in accordance with Beth Shammai. When is this so? (that a length of three or four fingers is required)? Ab initio (i.e., when the tzitzith are first inserted), but its remnants and lopped off (threads) may be of any (length).
("on the four corners of your garment": to exclude a garment of three, five, six, seven, or eight corners. Variantly: "gedilim" are white. Whence do I derive that I add a fourth of tcheleth? From (Ibid.) "and they shall put on the tzitzith of the corner a thread of tcheleth.")