This 4 sate switch is common.
If switch is pressed such that upper petrol light LED turn on, petrol fuel is selected and ur car will run on petrol.
Same is with CNG light, CNG fuel is selected.
Other two state will be used when you want to shift from one fuel to other.
If you are running on petrol and you want to convert to CNG, press the switch one time. Both light will be turned OFF. In this case, both fuel supply is OFF. Your car is still running because it is burning the petrol stored in the carburetor. Wait until car start jerking, at this time press the switch one more time. Now CNG light turn ON so car now running on CNG.
Now you are driving on CNG and you want to convert to petrol. Press the switch once. Both light (CNG, Petrol) will be on. So both supply ON. Now slightly press gas pedal wait a second or two. Again press switch and ur car will on petrol (CNG light ON).
Hopes it helps