Well they owe you $0.50/hr for every hour worked in the last 4 months. Thats a them mistake, not a you mistake, so dont feel bad.
Politely mention it to your boss or payroll. Dont assign blame, just tell them the issue. If the buisness is above board, they will apologize profusely and either offer to cut you a check today or by your next payday. If they are not above board, they will either fight you or bitch and moan before paying you. If that's the case, look for a new job. They will only do worse things to you in the future.