  A fourth generation of Standard Model (SM) fermions is usually considered
unlikely due to constraints from direct searches, electroweak precision
measurements, and perturbative unitarity. We show that fermion mass textures
consistent with all constraints may be obtained naturally in a model with four
generations constructed from intersecting D6 branes on a T^6/(Z_2 x Z_2)
orientifold. The Yukawa matrices of the model are rank 2, so that only the
third- and fourth-generation fermions obtain masses at the trilinear level. The
first two generations obtain masses via higher-order couplings and are
therefore naturally lighter. In addition, we find that the third and fourth
generation automatically split in mass, but do not mix at leading order.
Furthermore, the SM gauge couplings automatically unify at the string scale,
and all the hidden-sector gauge groups become confining in the range
10^{13}--10^{16} GeV, so that the model becomes effectively a four-generation
MSSM at low energies.
