  The description of quantum transport in a quadruple quantum-dot structure
(QQD) is proposed taking into account the Coulomb correlations and nonzero bias
voltages. To achieve this goal the combination of nonequilibrium Green's
functions and equation-of-motion technique is used. It is shown that the
anisotropy of kinetic processes in the QQD leads to negative differential
conductance (NDC). The reason of the effect is an interplay of the Fano
resonances which are induced by the interdot Coulomb correlations. Different
ways to increase the peak-to-valley ratio related to the observed NDC are
discussed.
