Why not just roll the dice and get another lens? Chances are, it will be perfect. If not, I can send it in. What do I have to lose?
Here, this should help:
I own it. It generally stays on my 50D (to the chagrin of my 17-55) and it has not disappointed me yet. (Of course, with the Bigmos it's having to share, but such are the quandaries of life). I really hate to hear about your situation with the F4, but maybe there's a reason for such things.
But that's just me...