Mq4 sensor doesnt detect natural gas but detects cigarette lighter gas only!
From what you're describing, it sounds as if the sensor has been damaged. The datasheet for this part gives warnings about a number of different conditions that can cause damage. You mention having tested the sensor with a cigarette lighter, and this seems like the most likely cause of your problem:


  2.2 Used in high gas concentration
  
  No matter the sensor is electrified or not, if it is placed in high gas concentration for long time, sensors characteristic will be affected. If lighter gas sprays the sensor, it will cause extremely damage.


Replace the sensor, and stop using a cigarette lighter for testing.