Should I tie Signal Gnd (pin 5) and Chassis Gnd (Pin 4) together on OBD-II port?
The two grounds come from different places. The signal ground comes from the PCM and the Chassis ground come from the body ground. 

If there are grounding issues in the car such as poor grounds there may be a voltage across them. Jumping then together would then induce a current loop. Best case scenario nothing happens. Worst case scenario wires get melted. 

Don't tie them together. Only use the chassis ground.