  It is shown that the existing four-loop result for the Bjorken polarized sum
rule for deep inelastic electron-nucleon scattering obtained within
perturbative Quantum Chromodynamics should be supplemented by the calculation
of the diagrams of the so called singlet type. We also give an explanation of
the interesting coincidence of two different classes of diagrams, one of the
non-singlet and one of the singlet type, contributing the
$\alpha_s^4$-approximation to the total cross-section of electron-positron
annihilation into hadrons.
