  We report photometric and radial velocity observations of the XO-4 transiting
planetary system, conducted with the FLWO 1.2m telescope and the 8.2m Subaru
Telescope. Based on the new light curves, the refined transit ephemeris of
XO-4b is $P = 4.1250828 \pm 0.0000040$ days and $T_c [BJD_TDB] = 2454485.93323
\pm 0.00039$. We measured the Rossiter-McLaughlin effect of XO-4b and estimated
the sky-projected angle between the stellar spin axis and the planetary orbital
axis to be $\lambda = -46.7^{\circ} ^{+8.1^{\circ}}_{-6.1^{\circ}}$. This
measurement of $\lambda$ is less robust than in some other cases because the
impact parameter of the transit is small, causing a strong degeneracy between
$\lambda$ and the projected stellar rotational velocity. Nevertheless, our
finding of a spin-orbit misalignment suggests that the migration process for
XO-4b involved few-body dynamics rather than interaction with a gaseous disk.
In addition, our result conforms with the pattern reported by Winn et al.
(2010, ApJL, 718, L145) that high obliquities are preferentially found for
stars with effective temperatures hotter than 6250~K.
