Solution: First time: Divide 28 biscuits into 2 parts, each part 14 biscuits, and put them on the left and right plates respectively. The tray rises with defective products;

The second time: take 14 biscuits with rising tray, divide them into 2 parts, 7 biscuits for each part, and put them on the left and right plates respectively, and those with rising tray are defective;

The third time: Divide six of the seven biscuits raised on the tray into two parts, three for each part, and put them on the left and right plates respectively. If the balance is balanced, the remaining one is defective; If the balance is unbalanced, the rising is flawed;

The fourth time: Take two of the three biscuits raised from the tray and put them on the left and right plates respectively. If the balance is balanced, the remaining one is defective; If the balance is unbalanced, the rising is flawed;

A: Generally speaking, defective products can be found at least four times.

So the answer is: 4.