  In this note we apply a 4-fold sum operation to develop an associativity rule
for the pairwise symplectic sum. This allows us to show that certain
diffeomorphic symplectic $4$-manifolds made out of elliptic surfaces are in
fact symplectically deformation equivalent. We also show that blow-up points
can be traded from one side of a symplectic sum to another without changing the
symplectic deformation class of the resulting manifold.
