Dear [Name],
We received your letter and were impressed by your dedication to filling out forms in triplicate. However, we regret to inform you that we are unable to process your request for a new form in triplicate due to a lack of quadruplicate forms.
We understand that this may cause you undue hardship, but we are willing to offer you an alternative solution: a form in quintuplicate. Please find attached the new form, which we have taken the liberty to fill out for you in anticipation of your prompt response.
Sincerely, [Name], Bureaucratic Red Tape Specialist