In ${BH}_4^{-}, {B}$ contributes three valence electrons and one each by four ${H}$ atoms and one negative charge sum up to give 4 valence shell electron pair to give $s p^3$ hybridization and tetrahedral structure.
In ${{NH}_4^{+}}$, ${N}$ contributes five valence electrons, four electrons by four ${H}$ atoms minus one electron for positive charge to give four valence shell electron pairs thus, $s p^3$ hybridization and tetrahedral shape.
