  All Globular Clusters (GCs) studied in detail so far host two or more
populations of stars. Theoretical models suggest that the second population is
formed from gas polluted by processed material produced by massive stars of the
first generation. However the nature of the polluter is a matter of strong
debate. Several candidates have been proposed: massive main-sequence stars
(fast rotating or binaries), intermediate-mass AGB stars, or SNeII. We studied
red giant branch (RGB) stars in the GC M4 (NGC 6121) to measure their chemical
signature. We confirm the presence of a bimodal population, first discovered by
Marino et al. (2008). The two groups have different C,$^{12}$C/$^{13}$C,N,O,Na
content, but share the same Li,C+N+O,Mg,Al,Si,Ca,Ti,Cr,Fe,Ni,Zr,Ba and Eu
abundance. Quite surprisingly the two groups differ also in their Y abundance.
The absence of a spread in $\alpha$-elements, Eu and Ba makes SNeII and AGB
stars unlikely as polluters. On the other hand, massive main-sequence stars can
explain the bimodality of Y through the weak s-process. This stement is
confirmed independently also by literature data on Rb and Pb. Observations
suggest that the mass of the polluters is between 20 and 30 M$_{\odot}$. This
implies a formation time scale for the cluster of 10$\div$30 Myrs. This result
is valid for M4. Other clusters like NGC 1851, M22, or $\omega$ Cen have
different chemical signatures and may require other kinds of polluter.
