  We present the results of a photometric search for variable stars in the core
of the Galactic globular cluster M4. The input data are a large and
unprecedented set of deep Hubble Space Telescope WFC3 images (large program
GO-12911; 120 orbits allocated), primarily aimed at probing binaries with
massive companions by detecting their astrometric wobbles. Though these data
were not optimised to carry out a time-resolved photometric survey, their
exquisite precision, spatial resolution and dynamic range enabled us to firmly
detect 38 variable stars, of which 20 were previously unpublished. They include
19 cluster-member eclipsing binaries (confirming the large binary fraction of
M4), RR Lyrae, and objects with known X-ray counterparts. We improved and
revised the parameters of some among published variables.
