Mercury Oil Tank Sensor
I also came through same kind of problem few days ago.
Mercury oil tank sensor. Low oil sending unit stuck on creating alarm from control panel. Mercury oil tank level sensor 4 beeps every 2 minutes. You can fix the problem by replacing wire to the horn for solving this particular problem or by replacing the sensor that measure the oil level in the motors. If you have a steady alarm going off but the oil is not low it is most likely the oil sending unit.
The engine 2 stroke oil tank has a low oil sensor in the bottom of it and can t be replaced without replacing the whole tank. This video will show you what causes this problem and how it can easily be corrected. The problem of faulty alarm module is commonly found among the mercury motors. So your mercury outboard is experiencing the common problem of the alarm beeping 4 times every 2 minutes.