Why it is necessory to assume sp3 hybridization in CH4?

Answer 1

In valence bond theory it is assumed the four electron pair bonds reside tetrahedrally about the carbon giving rise to the terahedral shape of the molecule.

sp3 hybridisation is "necessary", it replaces the s , px, py and pz orbitals with four orbitals of identical energy (degenerate) with lobes pointing to corners of a tetrahedron- the 4 electrons are then promoted to these orbitals - the hybridisation energy.