When a solute is added to a solvent, some properties are affected and these set of properties are called colligative properties. The properties depend on the amount of solute dissolved in a solvent. For freezing point depression, we calculate as follows:
ΔT(freezing point) = (Kf)m
ΔT(freezing point) = 1.86 °C kg / mol (4 mol / kg)
ΔT(freezing point) = 7.44 °C
Tf - T = 7.44 °C
T = -7.44 °C