Let G be a Sylow p-subgroup of the unitary groups GU (3, q 2 ), GU (4, q 2 ), the sympletic group Sp(4, q) and, for q odd, the orthogonal group O + (4, q). In this paper we construct a presentation for the invariant ring of G acting on the natural module. In particular we prove that these rings are generated by orbit products of variables and certain invariant polynomials which are images under Steenrod operations, applied to the respective invariant form defining the corresponding classical group. We also show that these generators form a SAGBI basis and the invariant ring for G is a complete intersection.