  We construct new supersymmetric SU(5) Grand Unified Models based on Z4 x Z2
orientifolds with intersecting D6-branes. Unlike constructions based on Z2 x Z2
orientifolds, the orbifold images of the three-cycles wrapped by D6-branes
correspond to new configurations and thus allow for models in which, in
addition to the chiral sector in 10 and 5-bar representations of SU(5), only,
there can be new sectors with (15 + 15-bar) and (10 + 10-bar) vector-pairs. We
construct an example of such a globally consistent, supersymmetric model with
four-families, two Standard Model Higgs pair-candidates and the gauge symmetry
U(5) x U(1) x Sp(4). In a N = 2 sector, there are 5 x (15 + 15-bar) and 1 x (10
+ 10-bar) vector pairs, while another N = 1 sector contains one vector-pair of
15-plets. The N = 2 vector pairs can obtain a large mass dynamically by
parallel D6-brane splitting in a particu- lar two-torus. The 15-vector-pairs
provide, after symmetry breaking to the Standard Model (via parallel D-brane
splitting), triplet pair candidates which can in principle play a role in
generating Majorana- type masses for left-handed neutrinos, though the
necessary Yukawa couplings are absent in the specific construction. Similarly,
the 10- vector-pairs can play the role of Higgs fields of the flipped SU(5),
though again there are phenomenological difficulties for the specific
construction.
