Lithium is one of the few elements synthesized during the primordial
Big Bang nucleosynthesis and its initial abundance in metal poor stars
sets constraints to the early nucleosynthesis in the Universe. The
discovery of the so-called Spite Plateau in unevolved, Population II
stars, has been interpreted as the signature of the pristine Li
abundance produced during the Big Bang. The very recent Li abundance
derived from the WMAP measurements is significantly higher (of at
least a factor of 3) than the Spite Plateau.  
I discuss Li and Fe abundances for 87 stars in the globular cluster
M4, obtained with the high-resolution spectrograph FLAMES@VLT, ranging
from the Turn-Off up to the RGB Bump. These results have been compared
with updated theoretical models with and without atomic diffusion
effects in order to disentangle the Li discrepancy and identify the
pristine Li abundance for the M4 stars.