<p>Seven different divalent transition metal sulfates (MgSO₄, CuSO₄, NiSO₄, CoSO₄, FeSO₄ · 7H₂O, ZnSO₄, and MnSO₄) and sodium carbonate (Na₂CO₃) were used as raw materials to synthesize a series of unary (MgCO₃, CoCO₃, ZnCO₃, FeCO₃, and MnCO₃) and quaternary (MgNiZnX)CO₃ (X = Mn, Fe, Co, Cu) carbon high-entropy materials under hydrothermal conditions. Microstructural studies of the obtained quaternary high-entropy carbonates are conducted, aiming to discuss the influence and causes of various transition metals on high-entropy phase formation, laying the experimental foundation for the preparation of high-entropy materials with different combinations of transition metals. Meanwhile, the phase and microscopic morphology of the samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that most quaternary compounds present a pure phase isomorphic with anhydrous carbonate. However, the presence of Cu as a component in the products hindered the formation of pure-phase high-entropy carbonates due to the influence of the Jahn–Teller effect. Additionally, each component's ionic radius and stabilization energy difference are the main reasons for the diffraction peak shift. Furthermore, variations in constituent elements played a crucial role in controlling particle morphology, with Fe favoring the formation of smooth spherical particles. At the same time, Cu exacerbated surface roughness on spherical particles, and the presence of Mn facilitated the formation of aggregates.</p>