  A general discussion is presented of the single logarithmic soft factor that
appears in two scale QCD observables in processes involving four partons. We
treat it as the ``fifth form factor'', accompanying the four collinear singular
Sudakov form factors attached to colliding and outgoing hard partons. The fifth
form factor is expressed in terms of the Casimir operators (squared colour
charges) of irreducible representations in the crossing $t$- and $u$-channels.
As an application we revisit the problem of large angle radiation in $gg\to gg$
and give a relatively simple solution and interpretation of the results. We
found an unexpected symmetry of the soft anomalous dimension under exchange of
internal and external variables of the problem whose existence calls for
explanation.
