  Nucleon matrix elements of various four-quark operators are evaluated in
quenched lattice QCD using Wilson fermions. Some of these operators give rise
to twist-four contributions to nucleon structure functions. Furthermore, they
bear valuable information about the diquark structure of the nucleon. Mixing
with lower-dimensional operators is avoided by considering appropriate
representations of the flavour group. We find that for a certain flavour
combination of baryon structure functions, twist-four contributions are very
small. This suggests that twist-four effects for the nucleon might be much
smaller than m_p^2/Q^2.
