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

Starspots and spin-orbit alignment in the WASP-4 exoplanetary system