I think you're looking for
this thread, but keep in mind that you probably won't re-sign him if he's filing for free agency no matter what. The guy I was able to re-sign said, "Unless you really surprise me with a great offer, I'm filing for free agency."
That said, I found that a 50 percent increase over his initial asking price (with a mutual option in the last year) did the trick. Your $4 million increase should be enough, if he's at all interested in returning.
What's his patience rating, btw? If it's below ~60, you're probably screwed.