Rick, you have the properties backwards AL4032 has a smaller expansion coefficient than AL2618. AL4032 is a super eutectic alloy that allows tighter piston to wall clearances than AL2618, but AL4032's strength falls off rapidly above 300 C temperature in comparison to AL2618. AL2618, on the other hand, has superior cooling capability because of the copper in the alloy and greater strength. You may also note that AL2618 was developed by Roll Royce during WWII for pistons in the reciprocating Aero engines.
Al4032 is a superior piston material for limited duty engines, think emissions. AL2618, on the hand, is still the preferred material for high output/ high duty cycle applications. Although there are even better aluminum composite alloys for pistons, these alloys are yet not allowed by the regulations in most of the racing organizations because of cost.