When traveling across different time zones, the number of hours in a day can change. This is due to the Earth`s rotation and the fact that different regions have different time zones.

Option 1 states that a day is added when crossing the equator from south to north. However, this is incorrect. The equator does not affect the number of hours in a day.

Option 2 suggests that a day is added when crossing the 180-degree longitude from east to west. This is also incorrect. Crossing the 180-degree longitude changes the date, but it does not add or subtract a day.

Option 3 states that a day is added when crossing the equator from north to south. This is incorrect. Again, crossing the equator does not affect the number of hours in a day.

Option 4 correctly states that a day is added when crossing the 180-degree longitude from west to east. This is because when traveling from west to east, you are moving ahead in time due to the Earth`s rotation. Therefore, the correct answer is option 4.