  Very deep images of the Galactic globular cluster M4 (NGC 6121) through the
F606W and F814W filters were taken in 2001 with the WFPC2 on board the HST. A
first published analysis of this data set (Richer et al. 2002) produced the
result that the age of M4 is 12.7 +/- 0.7 Gyr (Hansen et al. 2002), thus
setting a robust lower limit to the age of the universe. In view of the great
astronomical importance of getting this number right, we have subjected the
same data set to the simplest possible photometric analysis that completely
avoids uncertain assumptions about the origin of the detected sources. This
analysis clearly reveals both a thin main sequence, from which can be deduced
the deepest statistically complete mass function yet determined for a globular
cluster, and a white dwarf (WD) sequence extending all the way down to the 5
sigma detection limit at I~27. The WD sequence is abruptly terminated at
exactly this limit as expected by detection statistics. Using our most recent
theoretical WD models (Prada Moroni & Straniero 2002) to obtain the expected WD
sequence for different ages in the observed bandpasses, we find that the data
so far obtained do not reach the peak of the WD luminosity function, thence
only allowing one to set a lower limit to the age of M4 of ~9 Gyr. Thus, the
problem of determining the absolute age of a globular cluster and, therefore,
the onset of GC formation with cosmologically significant accuracy remains
completely open. Only observations several magnitudes deeper than the limit
obtained so far would allow one to approach this objective.
