  Calculational tools are provided allowing to determine general tree-level
scattering amplitudes for processes involving bosons and fermions in heterotic
and superstring theories in four space-time dimensions. We compute higher-point
superstring correlators involving massless four-dimensional fermionic and spin
fields. In D=4 these correlators boil down to a product of two pure spin field
correlators of left- and right-handed spin fields. This observation greatly
simplifies the computation of such correlators. The latter are basic
ingredients to compute multi-fermion superstring amplitudes in D=4. Their
underlying fermionic structure and the fermionic couplings in the effective
action are determined by these correlators.
