  We investigate the influence of higher twist corrections to deep inelastic
structure functions in the low-$Q^2$ and small-$x$ HERA region. We review the
general features of the lowest-order QCD diagrams which contribute to twist-4
at small-$x$, in particular the sign structure of longitudinal and transverse
structure functions which offers the possibility of strong cancellations in
$F_2$. For a numerical analysis we perform a twist analysis of the saturation
model which has been very successful both in describing the structure function
and the DIS diffractive cross section at HERA. As the main conclusion, twist 4
corrections are not small in $F_L$ or $F_T$ but in $F_2=F_L + F_T$ they almost
cancel. This indicates the limitation on the use of the DGLAP formalism at
small $x$ and $Q^2$. We point out that $F_L$ analysis needs a large twist-4
correction. We also indicate the region of validity of the twist expansion.
