What is the ph of the benzoic acid solution prior to adding sodium benzoate?

You are asked to prepare a pH = 4.00 buffer starting from 1.50 L of 0.0200 M solution of benzoic acid (C6H5COOH)and any amount you need of sodium benzoate (C6H5COONa).
What is the pH of the benzoic acid solution prior to adding sodium benzoate?