  The left-right models provide an explanation for the parity asymmetry in the
Standard Model (SM). To further understand the origin of the left-right
symmetry, we study a partial unification model based on SU(4)_W*U(1)_{B-L}
which can be broken down to the minimal left-right model either through the
Higgs mechanism in four dimensions or through the five-dimensional orbifold
gauge symmetry breaking with inner or outer automorphism. We scrutinize all
these breaking mechanisms and find that for the orbifold breaking in five
dimensions, the rank-reducing outer automorphism is better than the inner
automorphism and can make the low energy theory free of the U(1)_Z anomaly. For
the four-dimensional model with the Higgs mechanism, we study in detail both
its structure and its typical phenomenology. It turns out that this
four-dimensional scenario may predict some new phenomenology since the new
mirror fermions (which are introduced in order to fill the SM fermions into
SU(4)_W without anomaly) are preserved at low energy scale and mix with the SM
fermions. We also examine the gauge coupling unification in each case, and
discuss the possibility for unifying this partial unification group with the
Pati-Salam group SU(4)_{PS} to realize a grand unification.
