 Spin-only magnetic moment

The spin-only magnetic moment neglects orbital contributions and approximates $g$ by two: ${\mu }_{\mathrm{eff}}=g\sqrt{S(S+1)}{\mu }_B=\sqrt{n(n+2)}{\mu }_B$ for $n$ parallel unpaired electrons. Four unpaired electrons give $\sqrt{24}{\mu }_B\simeq 4.90{\mu }_B$.