Demand for a given commodity varies directly with the price of a substitute good.
For example, if the price of a substitute good (say, coke) increases, then demand for a given commodity (say, cane juice) will rise as cane juice will become relatively cheaper in comparison to coke. Hence 1 is correct.
A complementary good or service is an item used in conjunction with another good or service.
When the price of a good that complements good decreases, then the quantity demanded of one increases and the demand for the other increases.
For example, an increase in demand for cars will lead to an increase in demand for fuel. If the price of the complement falls, the quantity demanded of the other goods will increase. Hence 2 is incorrect.
​Inferior Good refers to an item that becomes less desirable as the incomes of its consumer's increases.
inferior goods are those whose price elasticity is negative.
As consumers’ incomes increase, they tend to decrease their purchases of inferior goods, opting for normal goods or luxury goods instead. Hence 3 is incorrect.
The quantity demanded of a good increase when its price falls is that the: lower price increases the real incomes of buyers, enabling them to buy more. Hence 4 is correct.