  In this paper, we classify connected graded quadratic Artin-Schelter regular
(AS-regular, henceforth) algebras of global dimension four that have a Hilbert
series the same as that of the polynomial ring on four generators and that map
onto a twisted homogeneous coordinate ring of a rank-two quadric. A twisted
homogeneous coordinate ring is a construction that was defined by Artin, Tate,
and Van den Bergh in \cite{ATV1, ATV2, AVdB1990} in the context of the
classification of AS-regular algebras of global dimension three. In
\cite{SV99,VVr}, the authors classified AS-regular algebras of global dimension
four that map onto the twisted homogeneous coordinate ring of a rank-three and
a rank-four quadric, respectively. We expand on their work to include the case
of coordinate rings of a rank-two quadric.
