Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) is a promising cathode material for sodium-ion batteries due to its structural stability and low cost, but its performance is often limited by low electronic conductivity and impurity formation. Here, we investigate the effects of Zn²⁺ substitution at Fe sites in NFPP, focusing on how Zn content and calcination atmosphere influence phase purity, local structure, and electrochemical properties. X-ray diffraction (XRD) shows that Zn²⁺ incorporation, particularly at higher concentrations, promotes the formation of electrochemically inactive maricite NaFePO₄ and Na₂ZnP₂O₇, reducing the phase purity and hindering Na⁺ transport. Complementary X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) reveal that Zn does not contribute to redox activity but modifies the Fe─O coordination environment, weakening the overall charge compensation compared to other transition-metal-doped NFPP cathodes. Galvanostatic cycling tests confirm that, despite good cycling stability, all Zn-doped NFPP cathodes exhibit inferior specific capacities compared to pristine NFPP. These results indicate that Zn²⁺ is not an effective capacity-enhancing dopant for NFPP under the studied conditions, while also highlighting the importance of dopant redox activity, structural compatibility, phase purity, and synthesis atmosphere in the design of sodium-ion battery cathodes.