  AIMS. We present abundance analysis based on high resolution spectra of 105
isolated red giant branch (RGB) stars in the Galactic Globular Cluster NGC 6121
(M4). Our aim is to study its star population in the context of the
multi-population phenomenon recently discovered to affect some Globular
Clusters. METHODS. The data have been collected with FLAMES+UVES, at the
ESO/VLT@UT2 telescope. Analysis was performed under LTE approximation for the
following elements: O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Ba, and NLTE
corrections were applied to Na and Mg. Spectroscopic data were coupled with
high-precision wide-field UBVI$_{\rm C}$ photometry from<EMAIL_ADDRESS>telescope and
infrared JHK photometry from 2MASS. RESULTS: We derived an average $\rm
{[Fe/H]}=-1.07\pm0.01$ (internal error), and an $\alpha$ enhancement of $\rm
{[\alpha/Fe]}=+0.39\pm0.05$ dex (internal error). We confirm the presence of an
extended Na-O anticorrelation, and find two distinct groups of stars with
significantly different Na and O content. We find no evidence of a Mg-Al
anticorrelation. By coupling our results with previous studies on the CN band
strength, we find that the CN strong stars have higher Na and Al content and
are more O depleted than the CN weak ones. The two groups of Na-rich, CN-strong
and Na-poor, CN-weak stars populate two different regions along the RGB. In the
$\rm {U}$ vs. $\rm {U-B}$ color magnitude diagram the RGB spread is present rom
the base of the RGB to the RGB-tip. Apparently, both spectroscopic and
photometric results imply the presence of two stellar populations in M4. We
briefly discuss the possible origin of these populations.
