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We report new metallicities for stars of Galactic globular cluster M4 usingthe largest number of stars ever observed at high spectral resolution in anycluster. We analyzed 7250 spectra for 2771 cluster stars gathered with the VLTFLAMES+GIRAFFE spectrograph at VLT. These medium resolution spectra cover by asmall wavelength range, and often have very low signal-to-noise ratios. Weattacked this dataset by reconsidering the whole method of abundance analysisof large stellar samples from beginning to end. We developed a new algorithmthat automatically determines the atmospheric parameters of a star. Nearly alldata preparation steps for spectroscopic analyses are processed on thesyntheses, not the observed spectra. For 322 Red Giant Branch stars with $V\leq 14.7$ we obtain a nearly constant metallicity, $<[{\rm Fe}/{\rm H}]> =-1.07$ ($\sigma$ = 0.02). No difference in the metallicity at the level of$0.01 ~\textrm{dex}$ is observed between the two RGB sequences identified by\cite{Monelli:2013us}. For 1869 Subgiant and Main Sequence Stars $V > 14.7$ weobtain $<[{\rm Fe}/{\rm H}]> = -1.16$ ($\sigma$ = 0.09) after fixing themicroturbulent velocity. These values are consistent with previous studies thathave performed detailed analyses of brighter RGB stars at higher spectroscopicresolution and wavelength coverage. It is not clear if the small meanmetallicity difference between brighter and fainter M4 members is real or isthe result of the low signal-to-noise characteristics of the fainter stars. Thestrength of our approach is shown by recovering a metallicity close to a singlevalue for more than two thousand stars, using a dataset that is non-optimal foratmospheric analyses. This technique is particularly suitable for noisy datataken in difficult observing conditions.

Atmospheric Parameters and Metallicities for 2191 stars in the Globular Cluster M4