The 450 °C isothermal sections of the Fe-Si-Sn-Zn quaternary system with Zn composition fixed at 70 and 93 at.% were determined experimentally using optical microscopy, scanning electronic microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffractometry (XRD). Five four-phase regions were identified in the 70 at.% Zn section, whereas no four-phase region was found in the 93 at.% Zn section. The liquid phase was found to be in equilibrium with almost all phases in the two sections, including the FeSn, FeSi, FeSi2, Γ1, δ, ζ, and α-Fe phases. The solubility of Sn in the ζ, FeSi, and FeSi2 phases was rather limited; however, the maximum solubility of Si in the FeSn phase was 0.5 wt.%. No quaternary compound was found in the study.