Orbital hybridization 1

There is a useful formula for determining the hybridization of a molecule :
If the No. of hybrid orbitals is $N$ , then,
$\displaystyle N=\frac{1}{2}[V_e+U-(c)]$
where,
$V_e=$ No. of electrons in Valence Shell of Central Atom,
$U=$ No. of Univalent Atoms around Central Atom,
$c=$ Charge (If it is an Ion)
If $N=2,3,4...$ then, hybridization is respectively $sp, sp^2, sp^3....$

So, for $BH_4^{-}$ ,
$V_e=3, U=4, c=-1$ ,
So, $N=\frac{1}{2}[3+4-(-1)]=\frac{1}{2}\times 8=4$
$\therefore$ hybridization is $\boxed{sp^3}$ .