  We give a relation between (right-handed) Dehn twists on the symplectic
mapping class group of a 4-dimensional Weinstein domain via holomorphic curve
techniques. A key ingredient of the construction is a solution to the
symplectic isotopy problem on symplectic submanifolds in Del Pezzo surfaces. In
the appendix, we provide an alternative proof of a relation between Dehn twists
and a fibered Dehn twist.
