  We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the
Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the
core and 19 more within the cluster half-mass radius. The limiting luminosity
of this observation is Lx~10e29 erg/sec for sources associated with the
cluster, the deepest X-ray observation of a globular cluster to date. We
identify 6 X-ray sources with known objects and use ROSAT observations to show
that the brightest X-ray source is variable. Archival data from the Hubble
Space Telescope allow us to identify optical counterparts to 16 X-ray sources.
Based on the X-ray and optical properties of the identifications and the
information from the literature, we classify two (possibly three) sources as
cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources
as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397
suggests a scaling of the number of active binaries in these clusters with the
cluster (core) mass.
