Sensing concentration of methane with MQ-4 methane sensor?
Actually, you are getting raw data from the sensor. A little bit of math should be done in order to calibrate it and based on the chart in datasheet, measure the real value in PPM.

The calibration basically helps you find R0, which is an internal resistance (along with RS form a voltage divider) that could be derived by doing calibration in a clean environment. After that, you use R0 to calculate a ratio and then look at the datasheet's chart to measure whatever gas you want since there is a curve describing it.

Please refer to this tutorial for more information. I have developed a similar application using MQ-4 before.

It is related to MQ-2, you may have noticed that all theses sensors have similar features and principle applications.