  We have derived the hierarchy of soliton equations associated with the
untwisted affine Kac-Moody algebra D^(1)_4 by calculating the corresponding
recursion operators. The Hamiltonian formulation of the equations from the
hierarchy is also considered. As an example we have explicitly presented the
first non-trivial member of the hierarchy, which is an one-parameter family of
mKdV equations. We have also considered the spectral properties of the Lax
operator and introduced a minimal set of scattering data.
