Accurately analyzing NMR and MRI diffusion experimental data relies on the theoretical expression used for signal attenuation or phase evolution. In a complex system, the encountered magnetic field is often inhomogeneous, which may be represented by a linear combination of z^n gradient fields, where n is the order. Additionally, the higher the order of the nonlinear gradient field, the more sensitive the phase variances are to differences in diffusion coefficients and delay times. Hence, studying higher-order fields has both theoretical and experimental importance, but this is a challenge for traditional methods. The recently proposed phase diffusion method proposed a general way to overcome the challenge. This method is used and demonstrated in detail in this paper to determine the phase evolution in a quadric field (n = 4). Three different types of phase evolution in the quadric gradient field are obtained. Moreover, a general signal attenuation expression is proposed to describe the signal attenuation for spin diffusion from the origin of the nonlinear gradient field. This approximation is based on the short gradient pulse (SGP) approximation but is extended to include the finite gradient pulse width (FGPW) effect by using the mean square phase. Compared to other forms of signal attenuation, such as Gaussian and Lorentzian, this method covers a broader range of attenuation, from small to relatively large. Additionally, this attenuation is easier to understand than the Mittag-Leffler function-based attenuation. The results, particularly the phase and signal attenuation expressions obtained in this paper, potentially advance PFG diffusion research in nonlinear gradient fields in NMR and MRI.