The ground-state properties of quartet quantum-dot arrays are studied using the numerical renormalization group (NRG) method with a four-site Hubbard model connected to two noninteracting leads. Specifically, we calculate the conductance and local charge in the dots from the many-body phase shifts, which can be deduced from the fixed-point eigenvalues of NRG. As a function of the on-site energy epsilon(d) which corresponds to the gate voltage, the conductance shows alternatively wide peaks and valleys. Simultaneously, the total number of electrons N-el in the four dots shows a quantized staircase behavior due to a large Coulomb interaction U. The conductance plateaus of the unitary limit emerging for odd N-el are caused by the Kondo effect. The valleys of the conductance emerge for even N-el, and their width becomes substantially large at half-filling. It can be regarded as a kind of the Mott-Hubbard insulating behavior manifesting in a small system. These structures of the plateaus and valleys become weak for large values of the hybridization strength Gamma between the chain and leads. We also discuss the parallel conductance for the array connected to four leads.