It's remotely possible they sent the machine back to you without the extra memory you paid for. If they have a really terrible problem repairing your machine, they reduce it to factory basic by pulling out all the fancy stuff.
Extra non-Apple Memory counts as fancy stuff.
Click on the Apple Logo, About This Machine and see how much physical memory you have. I think you need at least 1G for FCP4, but it might run OK on 512M. Some machines ship basic with 256M which won't run anything.
Koz