Ni–Fe–Ga-based Ferromagnetic Shape Memory Alloys (FSMAs) show considerable formability because of the presence of a disordered FCC γ-phase, but they lack in magnetocaloric property. Addition of Mn has been explored as a way to improve their magnetocaloric property. The current study presents a detailed structural and magnetization analyses of a two-phase ternary Ni₅₅Fe₁₉Ga₂₆ alloy and its quaternary counterparts obtained by partial replacement of Fe by Mn, Ni₅₅Fe_{19_x}Mn_xGa₂₆ (x = 2.5, 2.75, 3, 5, 10). Characterization of these alloys has been carried out using Optical and Scanning Electron Microscopy, Electron Probe Microanalysis, X-ray (XRD) and Neutron Diffraction (ND), Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC) and DC magnetization measurement. Ni₅₅Fe₁₉Ga₂₆ alloy shows predominantly non-modulated (NM) internally-twinned martensite, with traces of a modulated 14M martensite and the parent L2₁ phase along with the FCC γ-phase. Quaternary addition of Mn in partial replacement of Fe stabilizes14M martensite, drastically reduces the amount of γ-phase, keeps the martensitic transition temperatures unchanged, but raises T_C considerably. Magnetocaloric effect improves significantly with increasing Mn-content and a maximum value of ^_19.8 J/kg K for ΔS_M has been observed at 9 T for the alloy containing 10 at.% Mn.