  We report on a long-term optical monitoring of the neutron star X-ray binary
Centaurus X-4 performed during the last 13.5 years. This source has been in
quiescence since its outburst in 1979. Our monitoring reveals the overall
evolution of the accretion disc; we detect short-duration flares, likely
originating also in the disc, superimposed with a small-amplitude (< 0.1 mag)
ellipsoidal modulation from the companion star due to geometrical effects. A
long-term (~2300 days) downward trend, followed by a shorter (~1000 days)
upward one, is observed in the disc light curve. Such a rise in the optical has
been observed for other X-ray binaries preceding outbursts, as predicted by the
disc instability model. For Cen X-4, the rise of the optical flux proceeded for
~3 years, and culminated in a flux increase at all wavelengths
(optical-UV-X-rays) at the end of 2020. This increase faded after ~2 weeks,
without giving rise to a full outburst. We suggest that the propagation of an
inside-out heating front was ignited due to a partial ionization of hydrogen in
the inner disc. The propagation might have stalled soon after the ignition due
to the increasing surface density in the disc that the front encountered while
propagating outwards. The stall was likely eased by the low level irradiation
of the outer regions of the large accretion disc, as shown by the slope of the
optical/X-ray correlation, suggesting that irradiation does not play a strong
role in the optical, compared to other sources of emission.
