We discuss the accuracy of the present theoretical models of white dwarfs (WDs). We find that for ages higher than about 6-7 Gyr, the uncertainty in the predicted cooling times is of the order of 25%. The bulk of this uncertainty is due to the chemical profile in the Carbon/Oxygen core and the treatment of the conductive opacity. As a first application of our WD models, we chose the globular cluster M4, for which very deep HST images are available. Notwithstanding the very high quality, the quoted data do not reach the peak of the WD luminosity function, thence allowing one to set a lower limit to the age of M4 of ̃9 Gyr. No reliable upper limit can however be set. In order to achieve this goal one needs observations several magnitudes deeper than the limit obtained so far.