The correct answer for the unit of measurement of a light year is option 4, a very large distance. A light year is defined as the distance that light travels in one year in a vacuum. Since light travels at a constant speed of approximately 3 x 10^8 meters per second, in one year it will travel a distance of about 9.5 x 10^12 kilometers or roughly 5.88 x 10^12 miles.

It is important to note that a light year is a unit of distance, not time, temperature, or the age of the universe. Option 1, the age of the universe, is incorrect as the age of the universe is measured in years, not light years. Option 2, very small time intervals, is incorrect as light years measure distance, not time. Option 3, very high temperature, is incorrect as temperature is measured using different units such as Celsius or Fahrenheit. Therefore, the correct answer is option 4, a very large distance, because a light year is a unit used to express vast distances in space.