The focal length of a lens is inversely proportional to its power. The formula to calculate focal length (f) from lens power (P) in diopters is: f = 1/P. Therefore, for a lens with a power of 4D, the focal length would be f = 1/4 = 0.25 meters, or 25 centimeters. This means the lens will focus parallel rays of light at a point 25cm away from the lens.