We apply the functional renormalisation group to few-nucleon systems. Our
starting point is a local effective action that includes three- and
four-nucleon interactions, expressed in terms of nucleon and two-nucleon boson
fields. The evolution of the coupling constants in this action is described by
a renormalisation group flow. We derive these flow equations both in the limit
of exact Wigner SU(4) symmetry and in the realistic case of broken symmetry. In
the symmetric limit we find that the renormalisation flow equations decouple,
and can be combined into two sets, one of which matches the known results for
bosons, and the other result matches the one for fermions with spin degrees
only. The equations show universal features in the unitary limit, which is
obtained when the two-body scattering length tends to infinity. We calculate
the spin-quartet neutron-deuteron scattering length and the deuteron-deuteron
scattering lengths in the spin-singlet and quintet channels