  We classify and compute, by means of the six-dimensional embedding formalism
in twistor space, all possible three-point functions in four dimensional
conformal field theories involving bosonic or fermionic operators in
irreducible representations of the Lorentz group. We show how to impose in this
formalism constraints due to conservation of bosonic or fermionic currents. The
number of independent tensor structures appearing in any three-point function
is obtained by a simple counting. Using the Operator Product Expansion (OPE),
we can then determine the number of structures appearing in 4-point functions
with arbitrary operators. This procedure is independent of the way we take the
OPE between pairs of operators, namely it is consistent with crossing symmetry,
as it should be. An analytic formula for the number of tensor structures for
three-point correlators with two symmetric and an arbitrary bosonic
(non-conserved) operators is found, which in turn allows to analytically
determine the number of structures in 4-point functions of symmetric traceless
tensors.
