Hi, CS4
That means Photoshop CS4 (not Elements).
The Adobe platform swap link http://helpx.adobe.com/x-productkb/policy-pricing/order-product-platfo rm-language-swap.html says that transfers to a new platform are possible but only for current versions (currently CS6).
Swaps from earlier Windows versions to later Mac version require an upgrade - but upgrades from CS4 are no longer possible. You can only upgrade to CS6 from CS5.
Thus, the short answer is you'll have to re-purchase for the Mac - or join the Creative Cloud and install on Windows and/or Mac for the same monthly fee.