Experimental results are presented for a nonreciprocal phase shift of vertically polarized waves in four‐layer magneto‐optic rib waveguides. The structure consists of a high refractive index nonmagnetic layer of titanium oxide on top of a magneto‐optic film (a lanthanum and gallium substituted yttrium iron garnet film on a gadolinium gallium garnet substrate) to improve differential phase characteristics. The difference of phase constants for waves traveling in opposite directions is evaluated by observing the variation in the polarization coupled out of a straight waveguide when simultaneously exciting both horizontally and vertically polarized waves with equal amplitudes and phases. The phase shift difference varies with the thickness of the nonmagnetic film, and is larger than that of a previous three‐layer waveguide in the overlayer thickness range 0.15–0.45 &mgr;m. The maximum phase shift obtained is 2.5 times as large as that of uncoated waveguides.