  By employing D6-branes intersecting at angles in $D = 4$ type IIA strings, we
construct {\em four stack string GUT models} (PS-I class), that contain at low
energy {\em exactly the three generation Standard model} with no extra matter
and/or extra gauge group factors. These classes of models are based on the
Pati-Salam (PS) gauge group $SU(4)_C \times SU(2)_L \times SU(2)_R$. They
represent deformations around the quark and lepton basic intersection number
structure. The models possess the same phenomenological characteristics of some
recently discussed examples (PS-A class) of four stack PS GUTS. Namely, there
are no colour triplet couplings to mediate proton decay and proton is stable as
baryon number is a gauged symmetry. Neutrinos get masses of the correct sizes.
Also the mass relation
  $m_e = m_d$ at the GUT scale is recovered.
  Moreover, we clarify the novel role of {\em extra} branes, the latter having
non-trivial intersection numbers with quarks and leptons and creating scalar
singlets, needed for the satisfaction of RR tadpole cancellation conditions.
  The presence of N=1 supersymmetry in sectors involving the {\em extra} branes
is equivalent to the, model dependent, orthogonality conditions of the U(1)'s
surviving massless the generalized Green-Schwarz mechanism.
  The use of
  {\em extra} branes creates mass couplings that predict the appearance of
light fermion doublets up to the scale of electroweak scale symmetry breaking.
