  We present photometry of 4 transits of the exoplanet WASP-4b, each with a
precision of approximately 500 ppm and a time sampling of 40-60s. We have used
the data to refine the estimates of the system parameters and ephemerides.
During two of the transits we observed a short-lived, low-amplitude anomaly
that we interpret as the occultation of a starspot by the planet. We also find
evidence for a pair of similar anomalies in previously published photometry.
The recurrence of these anomalies suggests that the stellar rotation axis is
nearly aligned with the orbital axis, or else the star spot would not have
remained on the transit chord. By analyzing the timings of the anomalies we
find the sky-projected stellar obliquity to be -1_{-12}^{+14} degrees. This
result is consistent with (and more constraining than) a recent observation of
the Rossiter-McLaughlin effect. It suggests that the planet migration mechanism
preserved the initially low obliquity, or else that tidal evolution has
realigned the system. Future applications of this method using data from the
Corot and Kepler missions will allow spin-orbit alignment to be probed for many
other exoplanets.
