I don't have a definitive answer, but a few things come to mind.
Since you only have 4GB of RAM, it is very easy for your mac to start swapping to disk, as real RAM gets exhausted. Does restarting your mac free some space?
Also, try running OnyX and let it clear caches, etc.
One other thing: it has been reported a few times that having little hard drive space free somehow triggers situations where even that quickly goes.
I suggest using an external drive with plenty of space, rather than storing your material in the system drive (this is not just my recommendation, it is Apple's).
And upping your RAM to 8GB instead of 4GB should also help a lot with performance in FCP X.
I owe you five more points tried to mark you helpful and solved my problem but the second button disappeared. OnyX helped me identify that my startup volume still needed repair.
Now showing almost 70 Gbytes of free space!
So… for anyone else seeing this problem make sure when you run disk repair from your recovery volume that you run the repair on the volume NOT the drive.
I guess the moral of the story is never underestimate my capacity for ineptitude.
Thanks again Luis i was really starting to worry a bit there at this point.