  We present a minimal string-inspired supersymmetric $SU(4) \times SU(2)_L
\times SU(2)_R$ model, and provide a detailed analysis of the symmetry breaking
potential in this model, based on a generalisation of that recently proposed by
Dvali, Lazarides and Shafi. The model contains a global U(1) R-symmetry and
reduces to the MSSM at low energies. However it improves on the MSSM since it
explains the magnitude of its $\mu$ term and gives a prediction for $\tan \beta
\simeq m_t/m_b$. It also predicts an essentially stable proton, and contains
both `cold' and `hot' dark matter candidates. A period of hybrid inflation
above the symmetry breaking scale is also possible in this model. Finally it
suggests the existence of `heavy' charge $\pm e/6$ (colored) and $\pm e/2$
(color singlet) states.
