AU stands for "astronomical unit," which is a unit of measurement used in astronomy to represent distances within the solar system. One astronomical unit is defined as the average distance between Earth and the Sun, which is approximately 93 million miles or 150 million kilometers.
When you say that the distance from Earth to a planet in the solar system is 4 AU, it means that the planet is located at a distance that is four times the average distance between Earth and the Sun. In other words, the planet is approximately 372 million miles or 600 million kilometers away from Earth.
Using AU as a unit of measurement helps astronomers express distances within the solar system in a more convenient and standardized manner. It provides a reference point based on the average distance between Earth and the Sun, which is a fundamental quantity in our astronomical neighborhood.