Essentially, 4 blades can give you a finer finish or a faster mowing job (you can walk faster) provided your engine has enough power to drive them. Your blades have a rather short cutting length compared with some, so that might be an argument for running four - otherwise you'd have to walk a bit slower than with a longer blade at the same engine speed.
I think it is rather a minor point unless you are trying for a particularly fine finish, or you are in a hurry to get the job done and want to walk fast.
