That old version of FCP was released when most any system that would run FCP had an AGP graphics card. Your MacPro does not have an AGP card, so the old FCP won't launch. There have been work-arounds posted to fix that, but I don't know if it will work on version 4. Go through this thread to find the instructions to modify a .plist file in the FCP application. http://macosx.com/forums/mac-os-x-s...5936-final-cut-pro-asks-agp-card-startup.html
Apple did release a patch to fix that issue, but only for FCP version 5, and not for older versions, as far as I remember.