Answer: The fourth principle states that stable molecular orbitals are easiest to form when constructed out of atomic orbitals of similar energies. This means that 1s orbitals should combine with 1s orbitals and 2p orbitals should combine with 2p orbitals etc. to form the most stable molecular orbitals.
Explanation: In complex molecules, hybrid orbitals and valence bond theory can be used to describe σ bonding, and unhybridized π orbitals and molecular orbital theory can be used to describe π bonding.