Hybridisation of central metal ion in ${\left[\mathrm{Be}{\left(\mathrm{OH}\right)}_4\right]}^{-2}, {\left[\mathrm{Be}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]}^{+2}, {\left[\mathrm{Al}{\left(\mathrm{OH}\right)}_4\right]}^{-}$is

${\mathrm{OH}}^{-} \mathrm{and} {\mathrm{H}}_2\mathrm{O}$ are weak ligands;

Complexes with Coordination number '4' involving weak ligands are always tetrahedral;

Hybridisation will be ${\mathrm{sp}}^3$;