  We show that the optical counterpart of the X-ray source CX 1 in M4 is a 20th
magnitude star, located in the color-magnitude diagram on (or very close to)
the main sequence of the cluster, and exhibiting sinusoidal variations of the
flux. We find the X-ray flux to be also periodically variable, with X-ray and
optical minima coinciding. Stability of the optical light curve, lack of
UV-excess, and unrealistic mean density resulting from period-density relation
for semidetached systems, speak against the original identification of CX 1 as
a cataclysmic variable. We argue that the X-ray active component of this system
is a neutron star (probably a millisecond pulsar).
