Option (1) is incorrect: They can live together only with the permission of the court.
Option (2) is incorrect: They can live together with the permission of the court.
Option (3) is incorrect: No need to remarry, they can live together with the permission of the court.
Option (4) is correct: Under Section 10(2) of Hindu Marriage Act, 1955, on the application by petition of either party and on being satisfied of the truth of the statements made in such petition, rescind the decree of judicial separation.