Assuming the pH is lower by .4, and assuming the drop is totally due to injected CO2, no, it does not mean you have 40ppm. It means you have 2.5 (10^.4) times the CO2 you had before you started injecting it.
Or, assuming the pH is higher by .4, and assuming the rise is due to outgassing of CO2 from a previously CO2 injected tank, then it means you had 2.5 times the CO2 in your water prior to it all outgassing.
All you can tell is a relative value of CO2, 2.5 times. You cant tell an absolute value unless you know the beginning or ending levels of CO2.