In this letter, a low-profile four-port microstrip patch antenna (MPA) with wide isolation bandwidth and same polarization is proposed by using characteristic mode analysis. Firstly, the internal electric-field and modal significance (MS) of the shorted two-port MPA are theoretically studied. It shows that the null point of its internal electric-field can be reallocated to the unexcited feeding port by combining different modes. As such, the specific pins1, pins2, pins3 are loaded so as to gain three minimum poles for mutual coupling reduce. After that, the pins4 are further inserted to move these three minimum poles closely to each other, resulting in wideband isolation and broadside-beam. In addition, ring slots is etched to improve its input impedance. In final, our proposed four-port MPA is designed and tested. The results prove that its isolation bandwidth (|S₁₁| of -10 dB and isolation of above 20 dB) is obviously extended to about 16.2%, while keeping the broadside pattern with same polarization. Additionally, the MPA remains low-profile (0.06λ0), single-layer, and single-radiator.