  We investigate the frequency of two- and four-image gravitational lens
systems in the Jodrell-VLA Astrometric Survey (JVAS) and Cosmic Lens All-Sky
Survey (CLASS), and the possible implications for dark matter halo properties.
A simple lensing statistics model, which describes lens galaxies as singular
isothermal ellipsoids with a projected axis ratio distribution derived from the
surface brightness ellipticities of early-type galaxies in the Coma cluster, is
ruled out at the 98% level since it predicts too few four-image lenses (quads).
We consider a range of factors that may be increasing the frequency of radio
quads, including external shear fields, mass distributions flatter than the
light, shallow lensing mass profiles, finite core radii, satellite galaxies,
and alterations to the luminosity function for faint flat-spectrum radio
sources. We find that none of these mechanisms provide a compelling solution to
the quad problem on their own while remaining consistent with other
observational constraints.
