In this article, a novel design concept for bandwidth enhancement and mutual-coupling (MC) reduction of a single quad-port cross-slot antenna with simultaneously co-polarized and dual-polarized radiation patterns is proposed and designed. Initially, the first-, second-, and third-order modes (CM1-CM3) of a dual-port single-slot antenna with co-polarized pattern are theoretically studied by using a characteristic mode analysis (CMA). The results demonstrate that when one port is excited, the electric-field null of the antenna could be shifted to another unexcited port by combining these three modes, thus reducing the MC between two ports under a co- polarized pattern. Then, the short-ended bridges are loaded across the slot, resulting to move resonant frequencies of its CM1-CM3 closely to each other. After that, the microstrip lines are selected to excite the slot antenna for wide impedance matching. In addition, the linear slot is bent in order to reduce the electrical length. With these arrangements, our proposed quad-port cross-slot antenna has gained the desired co- polarized and dual-polarized radiation patterns with low MC. Finally, our designed antenna prototype is fabricated and measured. The results indicate that the isolation bandwidth ( S11 < -10 dB and MC < -17 dB) of our designed antenna is successfully extended to about 16.18% ranging from 4.83 to 5.68 GHz, becoming obviously widened compared to the traditional counterpart. Most importantly, both co- polarized and dual-polarized radiation patterns are independently realized under four ports while holding single layer, small thickness, single element, and stable radiation pattern simultaneously.