  We find and study a five-parameter family of four-dimensional coupled
Painlev\'e V systems with affine Weyl group symmetry of type $D_5^{(1)}$. We
then give an explicit description of a confluence from those systems to a
four-parameter family of four-dimensional coupled Painlev\'e III systems with
affine Weyl group symmetry of type $B_4^{(1)}$.
