The ${\mathrm{Y}}_{28}{\mathrm{Ti}}_{28}{\mathrm{Al}}_{24}{\mathrm{Co}}_{20}$ alloy undergoes metastable liquid phase separation in the under-cooled liquid state and subsequently solidifies into two different Y-rich and Ti-rich amorphous phases. Secondary phase separation occurs due to the supersaturation of the primary separated liquids as the temperature decreases. Depending on the degree of local undercooling, a wide range of length scale of the microstructure is observed. The characteristic length scale of the two amorphous phases is $\sim 250\mathrm{nm}$ near the air side of the ribbon, and $\sim 25\mathrm{nm}$ near the wheel side of the ribbon.