You may not install a Mac version on a PC, and CS4 is too old for Adobe to offer a platform swap
You MAY be able to get your serial number from Adobe if you can provide your old email, but I am not sure about that, since without some kind of proof, anyone could contact Adobe with a "dead" email and ask for a serial number
Link to buy CS6 via telephone http://www.adobe.com/products/cs6.html
According to what I was told by an employee, Mac newer than 10.7 must subscribe to the Cloud instead of buying CS6... and someone with Win10 must also subscribe to the Cloud since CS6 is not compatible (meaning the sales person will ask your operating system and will not sell you CS6 if it won't run on your computer... I don't know if that means only Win7 and you may not buy if you have Win8)
Please report back with your operating system if you are able to buy CS6