The 4 pins are Vcc (3-5VDC), GROUND, DO (Digital Out), and AO (Analog Out).  

Using the AO pin a voltage proportional to the vibration intensity can be read from an Arduino analog input.  

Using the DO pin, the potentiometer adjusts the set point.  If the vibration exceeds the set point the DO level goes LOW.

Specifications

• Fixed bolt hole for easy installation 
• PCB size: 15x30 mm aprox
• It use LM393 voltage comparator