  We prove that the examples by Smith and McMullen-Taubes provide infinitely
many counterexamples to one direction of Donaldson's 4-6 question and the
closely related Stabilising Conjecture. These are the first known
counterexamples. In the other direction, we show that the Gromov-Witten
invariants of two simply-connected closed symplectic $4$-manifolds, whose
products with $(S^2,\omega_{\text{std}})$ are deformation equivalent, agree. In
particular, when $b_2^+ \geq 2$, these $4$-manifolds have the same
Seiberg-Witten invariants. Furthermore, one can replace
$(S^2,\omega_{\text{std}})$ by $(S^2,\omega_{\text{std}})^k$ for any $k \geq 1$
in both results.
