In this paper, a four-parallel-coupled line is applied for the design of a novel filtering single-ended-to-balanced power divider. To facilitate the even- and odd-mode analysis of the symmetric structure, the proposed four-parallel-coupled line is split into two parallel coupled lines, and the closed-form design equations at f0 are given. Moreover, the coupled lines and open-circuited stubs between the four-parallel-coupled line and balanced output ports can realize performances of high-selectivity filtering and enhanced ultra-wideband common-/differential-mode suppression. By regulating two impedances of the OC stubs, three prototypes with different filtering bandwidths are designed and simulated. Among them, the circuit with the high-selectivity is fabricated and measured. Measurements agree well with simulations and ideal results, which verifies the validity of the theoretical derivations and circuit feasibility.