  The Transiting Exoplanet Survey Satellite (TESS) recently observed 18
transits of the hot Jupiter WASP-4b. The sequence of transits occurred 81.6
$\pm$ 11.7 seconds earlier than had been predicted, based on data stretching
back to 2007. This is unlikely to be the result of a clock error, because TESS
observations of other hot Jupiters (WASP-6b, 18b, and 46b) are compatible with
a constant period, ruling out an 81.6-second offset at the 6.4$\sigma$ level.
The 1.3-day orbital period of WASP-4b appears to be decreasing at a rate of
$\dot{P} = -12.6 \pm 1.2$ milliseconds per year. The apparent period change
might be caused by tidal orbital decay or apsidal precession, although both
interpretations have shortcomings. The gravitational influence of a third body
is another possibility, though at present there is minimal evidence for such a
body. Further observations are needed to confirm and understand the timing
variation.
