In this communication, a novel design approach to decrease the mutual coupling (MC) of a single dual-band four-port microstrip patch antenna (MPA) with a co-polarized radiation pattern is proposed via characteristic mode analysis (CMA). Initially, the traditional MPA under modes 1-4 is theoretically studied. This proves that the four modes maintain a high MC of approximately-2 dB. To address this issue, shorting pins 1 and 2 are inserted below the radiator, leading to a more than 12 dB MC reduction between the two ports along the $x$ -axis. After that, the MC between the two ports along the $y$ -axis is deeply studied. This proves that the electric field nulls of the MPA could be moved near unexcited ports and reduce the MC by combining modes 1 with 2 (or modes 3 with 4). Hence, shorting pin 3 is loaded for mode combination and low MC. In addition, shorting pin 4 is loaded to reshape the radiated fields in the upper band. Moreover, the impedance matching and the bandwidth are improved by cutting the ring slots. Finally, a four-port MPA is fabricated and tested. The results show that it has satisfactorily gained a low MC of below-15 dB between the four ports in dual bands. Meanwhile, it holds low-profile, single-layer, single-element, and co-polarized radiation pattern properties.