1. Calculate the molality of the solution:

* Molality (m) = moles of solute / kilograms of solvent

* Molality = 4 mol NaCl / 1 kg H₂O = 4 mol/kg

2. Calculate the boiling point elevation (ΔTb):

* ΔTb = Kb * i * m

* ΔTb = 0.51 °C/(mol/kg) * 2 * 4 mol/kg = 4.08 °C

3. Calculate the new boiling point:

* The normal boiling point of water is 100 °C.

* New boiling point = 100 °C + 4.08 °C = 104.08 °C

Therefore, the boiling point of water would increase by 4.08 °C to 104.08 °C when 4 mol of NaCl are added to 1 kg of water.