We are going to assume that the 4 M solution of KCl is a saturated solution. In other words, the maximum amount of KCl that can dissolve in water is 4 mol / 1 L of water.
Well, if we can turn that 4 mol / 1 L into MASS / 1 L, then we have our answer!
The key is to determine the molar mass of KCl.
Potassium has a molar mass of 39.0 g/mol.
Chlorine has a molar mass of 35.5 g/mol.
Therefore, KCl has a molar mass of 74.5 g/mol, or approximately 75 g/mol.
4 mol of KCl is 4x75 = 300 g
Therefore, 4 M KCl is equivalent to 300 g / L.