The Shulchan Aruch (24/1) writes that there is no obligation to wear a four cornered garment. Rather if you wearing one you must have tzitzis strings on it.

Yet we find that wearing tzitzis is a very important thing. It reminds you of all the mitzvos, you benefit great rewards (Shlchan Aruch 24/6), it saves you at a time of Hashem's anger (Menachos 41a), and there is a great punishment for those that don't keep this Mitzvah.

If Tzitzis is such an important Mitzvah, why is there no obligation to purchase a four cornered garment?