Tetravalency of carbon was proved by

In the case of hybridization, the 2S orbital electrons which were earlier paired are excited  $2p_z$ and become unpaired so that carbon can show Tetravalency. This could be only possible if the receiving orbital and the donating orbitals have same energy level. 

Through the process of hybridization, intermixing of orbitals takes place producing orbitals of same energy.