Dear Student
Equilibrium states that consumer consumes a good in such a manner that price will be equal to marginal utility or satisfaction derived by consuming an extra unit of the good . So for good Y we see that marginal utility is 4 and price of each unit is also 4 . So there is equilibrium in this case but as we move to good X we see that marginal utility is 3 and price of each unit is 4 . So the satisfaction obtained by consuming an extra unit is lesser than the price paid for the unit . So the consumer will lower the consumption of good X . As the consumer will stop consuming X or consume it in lesser quantities the marginal utility will automatically rise and match the price of the good . So the consumer will be in equilibrium .
Regards .