Final answer:
The refractive power of contact lenses may slightly differ from that of eyeglasses due to the distance the glasses sit from the eyes. Without specific formulae or additional data, the precise power cannot be determined for the librarian's contact lens prescription.
Step-by-step explanation:
The refractive power needed in contact lenses for a farsighted person who wears eyeglasses with a 4.02-D refractive power can be determined by considering the Lensmaker's equation and the fact that eyeglasses are typically worn about 1.5 cm away from the eyes. Given the additional distance of the eyeglasses from the eyes, the power of the contact lenses will be slightly different due to the vertex distance (the distance between the back of the eyeglass lens and the front of the cornea).
To determine the refractive power of the contact lens, one would need to conduct a calculation that considers the refractive power of the glasses, the vertex distance of the glasses, and the distance at which the person can comfortably read without correction. Since specific formula and additional information are not provided in this scenario, a precise value can not be given without the required data or the appropriate optometric equipment and examinations.