אורח חיים י׳Orach Chayim 10

Se’if 1א׳

טַלִּית שֶׁאֵין לָהּ ד' כְּנָפוֹת פְּטוּרָה, יֵשׁ לָהּ יוֹתֵר מֵאַרְבַּע חַיֶּבֶת, וְעוֹשֶׂה לָהּ אַרְבַּע צִיצִיּוֹת בְּאַרְבַּע כְּנָפֶיהָ הַמְרֻחָקוֹת זוֹ מִזּוֹ יוֹתֵר.

A tallit with less than four corners is exempt [from the requirement of tzitzit]. One with more than four corners requires tzitzit, and they should be put on the four corners that are the furthest from each other.

Se’if 2ב׳

יֵשׁ לָהּ אַרְבַּע, וְחָתַךְ אֶחָד בַּאֲלַכְסוֹן, וְעָשָׂהוּ שְׁנַיִם, הֲרֵי נַעֲשֵׂית בַּעֲלַת ה' וְחַיֶּבֶת.

If a tallit has four corners and one gets cut off crosswise to make two, this has now made it a five-cornered garment which requires tzitzit.

Se’if 3ג׳

כָּפַל קַרְנוֹת טַלִּיתוֹ וּקְשָׁרָם, אוֹ תְּפָרָם וְדוֹמֶה כְּאִלּוּ קִצְּעָן, וְאֵין לָהּ כְּנָפוֹת, אַף עַל פִּי כֵן לֹא נִפְטְרָה.

If one folds the corners of his tallit and ties them up or sews them and it looks like it has been trimmed and no longer has corners, even so it is not exempt [from tzitzit].