Diet pop contains less than 100 milligrams of sodium per 12-ounce can. That amount of sodium is less than one slice of bread. Diet colas contain less sodium than lemon-lime diet pop. Regular pop has less sodium than diet pop. Before aspartame, when sodium saccharin was used to sweeten diet beverages, the sodium content was slightly higher, but still under 100 milligrams per 12 fluid ounces.
I am not concerned with diet pop as a significant source of sodium unless you have been instructed to follow a low sodium diet of fewer than 1,000 milligrams per day which includes sodium in foods and added by you before eating. What I am concerned about is your drinking four to five cans per day. I would suggest limiting your intake of diet pop to less than two cans per day.