A friend asked me where he can find the serial no. on his Mercury 4 HP. I told him "I really don’t know because I only own OMC outboards". I don’t have access to the motor, but I have seen it, and if I have to guess it is from the 1980’s.
Hopefully, someone can point me in the right direction here.
It should be right on the swivel bracket, facing towards the rear of the motor. Prior to around 1986ish, they had a seven digit serial number. Then around that time period, they switched to an eight-place number that went: digit, letter, then 6 more digits.