If I am right , you are planning to buy the CS5 hence the article given by Mylenium would not help as Adobe provide the platform swap for the latest version.
You have to contact Adobe ....They will check the serial number of the CS4 is valid or not....and will register for you.
Once registered you can straight away place the order for the CS5 on the discounted price for MAC , there will be no problem weather you have the cs4 for mac or win
You should be bothered for your CS4 serial number because if you buy the upgrade of CS5 , when you will install the product it is gonna ask you the cs4 serial number .....hence get the serial number checked otherwise you have to buy the new software entirely.
Everything is dependent on CS4 serial numbr, buying CS5 is not the problem....you should have correct serial number of cs4 , get it checked.