  The zero-temperature phases of a generalized two-leg spin ladder with
four-spin exchanges are discussed by means of a low-energy field theory
approach starting from an SU(4) quantum critical point. The latter fixed point
is shown to be a rich multicritical point which unifies different competing
dimerized orders and a scalar chirality phase which breaks spontaneously the
time-reversal symmetry. The quantum phase transition between these phases is
governed by spin-singlet fluctuations and belongs to the Luttinger universality
class due to the existence of an exact U(1) self-duality symmetry.
