AU stands for Astronomical Unit, which is a unit of measurement used in astronomy to describe distances within the solar system. One Astronomical Unit is defined as the average distance between the Earth and the Sun, which is approximately 93 million miles or 150 million kilometers.
If the distance from Earth to a planet in the solar system is stated as 4 AU, it means that the planet is located four times farther from the Sun than Earth is. Since Earth is situated at an average distance of 1 AU from the Sun, a distance of 4 AU indicates that the planet in question is positioned at a distance four times greater than Earth's distance from the Sun. This would place the planet in the outer regions of the solar system, beyond the orbit of Mars but still within the range of the gas giants like Jupiter and Saturn.