code: def _compute_term_4(self, C, mag, R):
          
          return (
              (C['a16'] + C['a17'] * mag + C['a18'] * np.power(mag, 2) +
               C['a19'] * np.power(mag, 3)) * np.power(R, 3)
          )
docs: (a16 + a17.*M + a18.*M.*M + a19.*M.*M.*M).*(d(r).^3)