Context :

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 , the multi-fiber high resolution facility 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 those ( Na , Mg ) strongly affected by departure from LTE .
Spectroscopic data were coupled with high-precision wide-field UBVI _ { C } photometry from WFI @ 2.2m telescope and infrared JHK photometry from 2MASS .

Results : We derived an average [ Fe / H ] = -1.07 \pm 0.01 ( internal error ) , and an \alpha enhancement of [ \alpha / Fe ] = +0.39 \pm 0.05 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 .
The Na-rich group defines a narrow sequence on the red side of the RGB , while the Na-poor sample populate a bluer , more spread portion of the RGB .
In the U vs . U - B color magnitude diagram the RGB spread is present from 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 .

Conclusions :