After having gone through several Rubik's 4x4's that became worthless because they were too loose, I finally have one that is totally sweet.
What did I do to make it so sweet? Actually... NOTHING!!!!
This time, I didn't lube it; I just used it. I think the trick with Rubik's brand big cubes is NOT to lube them. They will loosen on their own with use, and once they get too loose there isn't much you can do to fix them. I don't know if this one will eventually become too loose as well, but at least I'm not hastening its demise.
By the way, they are pretty cheap in the stores right now. Maybe your best bet would be to purchase a new one and save the old one for parts.