  We compute in conventional dimensional regularisation the tree-level
splitting amplitudes for a quark parent in the limit where four partons become
collinear to each other. This is part of the universal infrared behaviour of
the QCD scattering amplitudes at next-to-next-to-next-to-leading order (${\rm
N^3LO}$) in the strong coupling constant. Further, we consider the iterated
limit when $m'$ massless partons become collinear to each other within a bigger
set of $m$ collinear partons, as well as the limits when one gluon or a
$q\bar{q}$ pair or two gluons become soft within a set of $m$ collinear
partons.

High Energy Physics - Phenomenology