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This paper presents an unconditionally stable split-step finite-difference time-domain (SS-FDTD) method with 4th order accuracy in time. Analytical proof of its unconditional stability is provided and numerical dispersion results are shown. Compared to the 2nd order SS-FDTD, the 4th order SS-FDTD yields a lower phase velocity error. Compared further to the 2nd order SS-FDTD with three iterations (for the same number of time marching steps), the 4th order SS-FDTD still achieves better numerical dispersion performance with sufficiently fine mesh.