  In this work, we provide important insights into the evolution of
half-metallicity in quaternary Heusler alloys. Employing {\it ab initio}
electronic structure methods we study 18 quaternary Heusler compounds having
the chemical formula CoX$^\prime$Y$^\prime$Al, where Y$^\prime$ = Mn, Fe; and
X$^\prime$ a 4$d$ element. Along with the search for new materials for
spintronics applications, the trends in structural, electronic, magnetic
properties and Curie temperature were investigated. We have made comparative
studies with the compounds in the quaternary series
CoX$^{\prime}$Y$^{\prime}$Si with X$^{\prime}$ materials from 3$d$ and 4$d$
transition metal series in the periodic table. We observe that the
half-metallic behaviour depends primarily on the crystal structure type based
on atomic arrangements and the number of valence electrons. As long as these
two are identical, the electronic structures and the magnetic exchange
interactions bear close resemblances. Consequently, the materials exhibit
identical electronic properties, by and large. We analysed the roles of
different transition metal atoms in affecting hybridisations and correlated
them with the above observations. This work, therefore, provides important
perspectives regarding the underlying physics of half-metallic behaviour in
quaternary Heusler compounds which goes beyond specifics of a given material.
This, thus, paves way for smart prediction of new half-metals. This work also
figures out an open problem of understanding how different ternary Heuslers
with different electronic behaviour may lead to half-metallic behaviour in
quaternary Heuslers with 4$d$ transition metal elements.
