The crystallization mechanism and solid-state phase transformations in Cu_46.5Zr₄₈Al₄Nb_1.5 metallic glass upon isokinetic and isothermal annealing are studied by using conventional and fast-scanning differential calorimetry, electric current flash-annealing, electromagnetic levitation, and in-situ high-energy synchrotron X-ray diffraction. Continuous-heating-transformation and time-temperature-transformation diagrams mapping the formation and evolution of the Cu₁₀Zr₇, B2 CuZr, τ₃ Zr₅₁Cu₂₈Al₂₁ and τ₄ Cu₂ZrAl phases in dependence on heating rate and temperature are constructed. Cu_46.5Zr₄₈Al₄Nb_1.5 metallic glass shows an increased propensity for the Cu₁₀Zr₇ phase formation compared to the parent Cu_47.5Zr_47.5Al₅ glass.