  We embed the minimal left-right model SU(2)_L*SU(2)_R* U(1)_{B-L} into an
SU(4)_W gauge group, and break the unified group via five-dimensional
S^1/(Z_2*Z_2) orbifolding. Leptons are fitted into SU(4)_W multiplets and
located on a symmetry preserving O brane, while quarks are placed onto an O'
brane where the symmetry is broken. This approach predicts \sin^2\theta_W=0.25
for the weak mixing angle at tree level and leads to a rather low weakly
(strongly) coupled unification scale of order 3*10^2 TeV (several TeV) with
supersymmetry, or as low as several TeV in the non-supersymmetric case. Another
symmetry breaking chain with the low-energy gauge group
SU(2)_L*U(1)_{3R}*U(1)_{B-L} can also give rise to a weak mixing angle
\sin^2\theta_W=0.25 at tree level after gauge symmetry breaking by orbifolding.
Such theories with low-scale unification have interesting phenomenological
consequences.
