Is zakat obligatory on merchandise that is sold completely within four months, and thus a hawl (one year) does not pass on it? And what is the amount of this zakat?

The zakat on trade goods is the zakat on the money with which the goods were purchased. It becomes obligatory when a year has passed over it (the money) and it reaches the nisab (minimum threshold), either by itself or when combined with other money or trade goods. The nisab for banknotes is the equivalent of 85 grams of gold or 595 grams of silver, and the amount due is a quarter of a tenth (2.5%). If you own the money and a year has passed over it, zakat becomes obligatory upon selling the goods.