Every time I run Motion 4, it shows me the startup screen, I choose a new project and it just crashes. I reinstalled it but that didn't work. I'm on a MBP with an 2.53 GHz Intel Core 2 Duo and 4GB of RAM.
Have you tried deleting all the preferences of motion. If this does not work, fix you permissions on you Mac and see if this resolves the problem. Usually reinstalling the program resOlves this issues. If all fails try creating new user on your computer than try log on and try to start motion from that user. If all fails again that is is a issue with your system not FCP.