Correction: I did not test this while writing and missed two spots. You also need to edit Contract.inc.json and duplicate the line for the last target, then replace args['targets'] < 4 with args['targets'] < 5 and index=3:i with index=4:i. Also replace a few characters in the id, it just needs to be different from the other objectives. Furthermore, you need to replace 1 with 5 in the corresponding 1.framework.json file for the mission, just besides the 1.entity.json. If you want a mission picture, you also need to add that with the correct number.
I still feel that 5 is too much, though.