ADC value (4000 to 20000) to (0-400) milibar conversion
If your ADC goes from 4000 to 20000 it means you have 16000 point to mesure 400mbar.

Then the resolution of your mesure is:

resolution= 400/16000 = 0.025mbar. 

And you have an offset of 4000 point.

Then your value in mbar is : P = (ADCvalue - offset) * resolution.