On the hyperfine structure of the triplet $n^{3}S-$states of the four-electron atoms and ions

Hyperfine structures of the triplet $n^{3}S-$states in the four-electron Be-atom(s) and Be-like ions are considered. It is shown that to determine the hyperfine structure splitting in such atomic systems one needs to know the triplet electron density at the central atomic nucleus ${\rho }_T(0)$. We have developed the procedure which allows allows one to determine such an electron density ${\rho }_T(0)$ for arbitrary four-electron atoms and ions.