This question would be better for the FCP forums, but I'll give it a shot.
I don't believe you can have FCP5 and 4.5 exist on the same partition. When you install FCP5 on a system that has FCP4.5 on it, FCP4.5 is automatically deleted. FCP5 has many new extensions and relies on too many libraries to be compatible with those from 4.5. So, if you want to have FCP5 and 4.5 exist on the same machine, you need to either partition your hard drive or install a second drive, and then install OS X on the second partition or drive and then FCP5 onto that disk. You could then try to copy FCP4.5 to the new partition or run FCP 4.5 from the OS with FCP5 installed and see if that works, but I doubt it.