The 4th alphanumeric will be a letter. It's order in the alphabet is the month of manufacture. The letter "I" is skipped, not used, due to the possibility of it being mistaken for the numeral "1". For example, in the made up serial number "L09H1234", the 4th character is "H", which is the 8th letter, that means the 8th month - August. This style of serial number has been used since, I think, the late 1970's.