The Co−Ni−Ti−V quaternary phase diagrams within the Co−Ni-rich region were investigated using the electron probe X-ray micro-analyzer (EPMA) and X-ray diffraction (XRD). Three isothermal sections corresponding to the Co−10Ni−Ti−V, Co−15Ni−Ti−V, and Co−20Ni−Ti−V quaternary systems at 1000 °C were experimentally established. The results indicate that increasing Ni content markedly broadens the γ (α-Co) and γ′ (Co₃Ti) two-phase regions. Based on the Co−Ni−Ti−V phase diagram, alloys with high γ′ solvus temperature were designed, and their comprehensive properties, including γ′ coarsening behavior and mechanical properties, were thoroughly investigated. Compared to Co−Ti−based superalloys, the Co−20Ni−10Ti−10V alloy exhibits lower coarsening rates of γ′ precipitates and γ/γ′ lattice mismatch. Notably, it possesses exceptional high-temperature mechanical properties, with a yield strength of 508 MPa at 1000 °C. This superior performance is primarily attributed to the presence of a high density of stacking fault shear.