We have employed a thermodynamic-computation approach to identify the composition of the Zr-Cu-Ni-Al alloy system exhibiting a two-liquid miscibility phase equilibrium in the liquid-temperature region, which tends to favor the occurrence of the liquid-phase separation. Guided by these calculated diagrams, a Zr-based alloy with a 10 at.% Al is designed, and its bulk-metallic glass (BMG) is prepared successfully by the copper-mould suction casting. A heterogeneous microstructure characterized by the existence of phase-separated regions with several to decades micrometers in size forms in the BMG. Under uniaxial compressive loading, the designed Zr-based BMG demonstrates the continuous "work hardening" and remarkable macroscopic plastic strain at room temperature. The improvement of mechanical properties is attributed to the unique glassy structure correlated with both the heterogeneous microstructure and the micro-scaled phase separation, leading to the extensive shear-band formation, interaction, and multiplication.