  We use the observed optical-UV and X-ray emission spectrum of Cen X-4 during
quiescence to constrain models for the accretion flow in this system. We argue
that the optical-UV emission is not due to an optically-thick quiescent
accretion disk, nor due to synchrotron emission from an Advection-Dominated
Accretion Flow (ADAF). Emission from the bright spot could account for the
observed optical-UV component if the mass transfer rate in Cen X-4 is >~
2.10^16 g/s. Although the presence of an ADAF around the neutron star leads to
Compton upscattering of the soft X-ray photons radiated from the stellar
surface, we find that this process alone cannot account for the power law
component seen in the quiescent X-ray spectrum of Cen X-4 and other X-ray
transients containing neutron stars; this result is independent of whether the
source of soft photons is incandescent thermal emission or accretion-powered
emission. We conclude that, in models which invoke the presence of an ADAF and
a propeller effect for the quiescence of X-ray transients containing neutron
stars, the intrinsic emission from the ADAF must contribute very little to the
optical-UV and X-ray emission observed. If these ADAF+propeller models are
correct, the X-ray power law component observed must arise from regions where
the gas impacts the neutron star surface. Variability studies could greatly
help clarify the role of the various emission mechanisms involved.
