  I report systematic first-principle calculations of the quaternary Heusler
alloys like Co$_2$[Cr$_{1-x}$Mn$_x$]Al, Co$_2$Mn[Al$_{1-x}$Sn$_x$] and
[Fe$_{1-x}$Co$_x$]$_2$MnAl. I show that when the two limiting cases (x=0 or 1)
correspond to a half-metallic compound, so do the intermediate cases. Moreover
the total spin moment $M_t$ in $\mu_B$ scales linearly with the total number of
valence electrons $Z_t$ (and thus with the concentration $x$) following the
relation $M_t=Z_t-24$, independently of the origin of the extra valence
electrons, confirming the Slater-Pauling behavior of the normal Heusler alloys.
Finally I discuss in all cases the trends in the atomic projected DOSs and in
the atomic spin moments.
