1. Calculate the moles of NaCl:

* Molar mass of NaCl: 58.44 g/mol (22.99 g/mol for Na + 35.45 g/mol for Cl)

* Moles of NaCl: (125 g NaCl) / (58.44 g/mol) = 2.14 mol NaCl

2. Calculate the molarity:

* Molarity (M) = moles of solute / liters of solution

* Molarity = (2.14 mol NaCl) / (4.00 L solution) = 0.535 M

Therefore, the molarity of the NaCl solution is 0.535 M.