Consider a plano-convex lens that is flat on one side, has a 15.0 cm radius of curvature on the other lens, and is composed of magneto-optical plastic. This plastic has an index of refraction of 1.550. The lens is being used to form an image of an object situated 60.0 cm from the lens on a viewing screen. If a magnetic field is applied to the plastic, causing the index of refraction to decrease, what change in the refractive index would be necessary to shift the image 4.0 cm further away from the lens?