We investigate electronic states upon doping (i.e., by varying the chemical potential) for a quarter-filled one-dimensional electron system with a dimerization, on-site and nearest-neighbor repulsive interaction. By applying the renormalization group method to the bosonized Hamiltonian, the metal-insulator transition followed by commensurate-incommensurate transition is obtained for Mott insulating region as well as charge ordered insulating regime. We discuss the phase diagram on the plane of the chemical potential and the interaction.