If the parties have not designated the obligation to be discharged by appropriation, it shall be done in accordance with the provisions of the following subparagraphs:
1. If some of the obligations are due and others are not due, it shall be appropriated for the performance of the obligation which is due;
2. If all the obligations are due or all are not due, it shall be appropriated for the performance of the obligation the discharge of which is most advantageous to the obligor;
3. If the advantage to the obligor is equal, it shall be appropriated for the performance of the obligation which first became due or will become due;
4. If the facts mentioned in the preceding two subparagraphs are equal, it shall be appropriated for the performance of each obligation in proportion to the amount of each obligation.