  In a Globular Cluster (GC), the mass loss during the red-giant branch (RGB)
phase and the helium content are fundamental ingredients to constrain the
horizontal branch (HB) morphology. While many papers have been dedicated to the
helium abundance in the different stellar populations, small efforts have been
done to disentangle the effects of mass loss and helium content. We exploit the
nearby GC NGC6121 (M4), which hosts two well-studied main stellar populations,
to infer both helium and RGB mass loss. We combine multi-band Hubble Space
Telescope photometry of RGB and main sequence (MS) stars of M4 with synthetic
spectra to constrain the relative helium content of its stellar populations. We
find that the second generation stars in M4 is enhanced in helium mass fraction
by $\rm \Delta Y = 0.013 \pm 0.002$ with respect to the remaining stars that
have pristine helium content. We then infer the mass of the HB stars by
searching for the best match between the observations and HB populations
modelled assuming the helium abundance of each population estimated from the
MS. By comparing the masses of stars along the HB, we constrain the mass loss
of first- and second-generation stars in M4. We find that the mass lost by the
helium enriched population is $\sim 13$% larger than the mass lost by the first
generation stars ($\rm \Delta \mu = 0.027 \pm 0.006 \ M_\odot$). We discuss the
possibility that this mass loss difference depends on helium abundance, the
different formation environment of the two generations, or a combination of
both.
