If 4 g of NaOH dissolves in 36 g of H2O,

Mole fraction of H2O = No; of moles of H2O/ Total no: of moles (H2O+NaOH)

No: of moles of H2O = 36/18=2moles

No: of moles of NaOH = 4/40=0.1mol

Total no: of moles = 2+0.1= 2.1

Mole fraction of H2O = 2/2.1 = 0.952

Mole fraction of NaOH = 0.1/2.1 = 0.048

Mass of solution = Mass of H2O + Mass of NaOH = 36+4=40G

Volume of the solution = 40/1 = 40mL

Molarity = 0.1/0.04 = 2.5M