Solid solutions (SS) with a quasi-binary cross-section of a four-component system of the composition (1?x)(Na0.5K0.5)NbO₃–xPb(Ti0.5Zr0.5)O₃, based on compositions with fundamentally different physical responses (Na, K)NbO₃ (KNN), Pb(Ti, Zr)O₃ (PZT), have been prepared by a two-stage solid-phase synthesis followed by sintering using conventional ceramic technology. The influence of thermal cycling on the dielectric properties of ceramic SS with x=0.00,…,1.00 has been studied. It has been shown that thermally induced fatigue does not have a significant effect on the Curie temperature and the diffusing of the phase transition (PT) of SS containing only extreme components. It has been found that when the concentration of PZT in the studied system is varied, the ε′/ε0(T) dependencies take on a form characteristic of ferroelectrics with a diffuse PT. When the repolarisation cycles are varied, the diffusing parameter and the Curie and Burns temperatures show a wave-like behavior. The observed effects are explained by the interaction of different defect types. A conclusion is drawn on the possible practical applications of the studied SS.