Equinox occurs when the sun is vertically above the Equator. During an equinox, which happens twice a year, the sun is directly overhead at the equator. This means that the length of day and night are almost equal all around the world.

Option 1, the Tropic of Capricorn, is incorrect. The Tropic of Capricorn is located at 23.5 degrees south of the Equator and marks the southernmost point where the sun can be directly overhead.

Option 2, the Tropic of Cancer, is incorrect. The Tropic of Cancer is located at 23.5 degrees north of the Equator and marks the northernmost point where the sun can be directly overhead.

Option 3, the Poles, is incorrect. The poles are the areas located at the extreme north and south of the Earth and are not affected by equinoxes.

Therefore, the correct answer is option 4, the Equator, where the sun is vertically above during an equinox.