  Hyperfine structures of the triplet $n^3S-$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.
