Elevation in boiling point for \( 1.5 \) molal solution of glucose in water is \( 4 \, \mathrm{K} \). The depression in freezing point for \( 4.5 \) molal of glucose in water is \( 4 \, \mathrm{K} \). The ratio of molal elevation constant to molal depression constant \( \left( \frac{K_b}{K_f} \right) \) is ______.