4. Within the d-orbitals.

Explanation:
Colour in transition series metal compounds arises due to two types of transitions. they are:
(a) Charge transfer transitions [Ligand-to-metal charge transfer (LMCT), Metal-to-ligand charge transfer]
(b) d−d transition
In transition metal complexes, all the d− orbitals do not possess same energy. So, electrons can jump from a lower energy d− orbital to a higher energy d− orbital, by absorbing energy. When it returns to the ground state, excess energy is released, and a corresponding wavelength is found in the visible region.