First, put two bags of sugar at both ends of the balance.

1. If their weights are different, then the defective products are in these two bags. Take a bag and put any of the other three bags.

1. 1 If the weight is the same, the defective product is in the take-away bag.

1.2 If the weight is different, the defective product is the bag left in the balance.

If the weight is the same, then the defective products are in the remaining 3 bags.

2. 1 Take 2 out of the remaining 3 bags and put them at the 2 ends of the balance.

2. 1. 1 If the weight is the same, the defective product is the remaining bag.

2. 1.2 If the weight is different, repeat 1.