CPR Compatibilty

Hi guys,
I am now running C6 on my mastering Mac, and still write my primary projects on this machine.
But, I have C4 on two other machines that are networked.
All of my projects are saved in C4 format. Any saving on C6 will upgrade the CPR format, but I don’t want this to happen. I need a Cubase format that are interchangeable between platforms, without compromise.
Is this a problem that I have to live with?
I can’t afford to update both Mac’s to C6