  The determination of twist-4 corrections to the structure functions of
polarized $e(\mu)N$ scattering by QCD sum rules is reviewed and critically
analyzed. It is found that in the case of the Bjorken sum rule the twist-4
correction is small at $Q^2 > 5 GeV^2$. However, the accuracy of the today
experimental data is insufficient to reliably determine $\alpha_s$ from the
Bjorken sum rule. For the singlet sum rule -- $p+n$ -- the QCD sum rule gives
only the order of magnitude of twist-4 correction. At low and intermediate
$Q^2$ the model is presented which realizes a smooth connection of the
Gerasimov-Drell-Hearn sum rules at $Q^2 = 0$ with the sum rules for
$\Gamma_{p,n}(Q^2)$ at high $Q^2$. The model is in a good agreement with the
experiment.
