  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 non-interacting 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 peak and valley. Simultaneously, the total
number of electrons $N_{\rm el}$ in the four dots shows a quantized stair case
behavior due to a large Coulomb interaction $U$. The conductance plateaus of
the Unitary limit emerging for odd $N_{\rm el}$ are caused by the Kondo effect.
The valleys of the conductance emerge for even $N_{\rm el}$, 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.
