  The generalized Drinfel'd-Sokolov hierarchies studied by de
Groot-Hollowood-Miramontes are extended from the viewpoint of Sato-Wilson
dressing method. In the A_1^(1) case, we obtain the hierarchy that include the
derivative nonlinear Schr\"odinger equation. We give two types of affine Weyl
group symmetry of the hierarchy based on the Gauss decomposition of the A_1^(1)
affine Lie group. The fourth Painlev\'e equation and their Weyl group symmetry
are obtained as a similarity reduction. We also clarify the connection between
these systems and monodromy preserving deformations.
