Abstract: Solid-state impedance spectroscopy (SS-IS) was used to investigate the influence of structural modifications resulting from the addition of Nb₂O₅ on the dielectric properties and relaxation processes in the quaternary mixed glass former (MGF) system 35Na₂O–10V₂O₅–(55−x)P₂O₅–xNb₂O₅ (x = 0–40, mol%). The dielectric parameters, including the dielectric strength and dielectric loss, are determined from the frequency and temperature-dependent complex permittivity data, revealing a significant dependence on the Nb₂O₅ content. The transition from a predominantly phosphate glass network (x < 10, region I) to a mixed niobate–phosphate glass network (10 ≤ x ≤ 20, region II) leads to an increase in the dielectric parameters, which correlates with the observed trend in the direct-current (DC) conductivity. In the predominantly niobate network (x ≥ 25, region III), the highly polarizable nature of Nb⁵⁺ ions leads to a further increase in the dielectric permittivity and dielectric strength. This is particularly evident in Nb-40 glass-ceramic, which contains Na₁₃Nb₃₅O₉₄ crystalline phase with a tungsten bronze structure and exhibits the highest dielectric permittivity of 61.81 and the lowest loss factor of 0.032 at 303 K and 10 kHz. The relaxation studies, analyzed through modulus formalism and complex impedance data, show that DC conductivity and relaxation processes are governed by the same mechanism, attributed to ionic conductivity. In contrast to glasses with a single peak in frequency dependence of imaginary part of electrical modulus, M″(ω), Nb-40 glass-ceramic exhibits two distinct contributions with similar relaxation times. The high-frequency peak indicates bulk ionic conductivity, while the additional low-frequency peak is associated with the grain boundary effect, confirmed by the electrical equivalent circuit (EEC) modelling. The scaling characteristics of permittivity and conductivity spectra, along with the electrical modulus, validate time-temperature superposition and demonstrate a strong correlation with composition and modification of the glass structure upon Nb₂O₅ incorporation.