fourth year
The serial number usually has the manufacture date in it. Some use month and year numbers, others use a letter for the month with the year after it, and still others post the week of manufacture with the year following it. When they do not use one of these methods then the date is in a code that you need their conversion tables to figure it out. The warrranty period is usually in the model number 5, 6, V,, etc.
