Non-plan expenditure refers to the expenses that the government incurs not related to any planned economic development.

Option 1: Subsidies (I) are monetary help provided by the government to persons or groups, not linked with any development project hence it`s a non-plan expenditure. Interest payments (II) are incurred on the loans taken by the government to fund its expenditures and are non-plan.

Option 2: Subsidies (I) have been discussed already. Defence expenditure (III) are not associated with economic growth or development, hence, non-plan.

Option 3: Interest payments (II) and maintenance expenditure for the infrastructure created in the previous plans (IV) are both non-plan expenditures as they do not contribute to any current development plan.

Option 4 includes all (I, II, III, IV) the expenditures discussed above which come under non-plan expenditure.

Therefore, the correct answer is option 4, that is, all of them (I, II, III, and IV) are non-plan expenditures.