I think you only need to install your CS6 software. During the installation process it will tell you it is an upgrade package and it can't find any previous versions installed. At this point it will ask you for your old (CS3 or CS4) serial number to input. After that the installation will proceed as usual.
Don't install anything except CS6. It will prompt you for your serial number...just enter the one for CS4.
If you did have the previous version installed, you would not be prompted for the serial number, but trust me that you DO NOT want to install the older versions unless you need them on your computer for some bizarre reason.
Also, if you ever did need the older versions, you wouldn't need to install them in XP Pro mode, it should install just fine as normal in Win7x64. It will of course run as a 32-bit app under CS3 or CS4, but the installer and Win7 are smart enough to figure it all out 99% of the time.
Magnus, thank you very much, will do.
Christian, thank you, will do.