The expression "4 x 10^-3" is a way to represent a number in scientific notation. It means 4 multiplied by 10 raised to the power of -3. This indicates that the decimal point in the number 4 should be moved three places to the left, resulting in 0.004. Scientific notation is often used to simplify the representation of very small or very large numbers. In this case, "4 x 10^-3" is equivalent to 0.004, which can also be expressed as 4 millimeters or 4 grams in certain contexts. This notation is particularly useful in fields like science, engineering, and finance, where precise measurements are essential.